The U.S. and Britain struck 18 Houthi targets in Yemen on Saturday, answering a recent surge in attacks by the Iran-backed militia group on ships in the Red Sea and Gulf of Aden, including a missile strike this past week that set fire to a cargo vessel.

According to U.S. officials, American and British fighter jets hit sites in eight locations, targeting missiles, launchers, rockets, drones and air defense systems. The officials spoke on condition of anonymity in order to provide early details of an ongoing military operation.

This is the fourth time that the U.S. and British militaries have conducted a combined operation against the Houthis since Jan. 12. But the U.S. has also been carrying out almost daily strikes to take out Houthi targets, including incoming missiles and drones aimed at ships, as well as weapons that were prepared to launch.

The U.S. F/A-18 fighter jets launched from the USS Dwight D. Eisenhower aircraft carrier, which is currently in the Red Sea, officials said.

“The United States will not hesitate to take action, as needed, to defend lives and the free flow of commerce in one of the world’s most critical waterways,” said U.S. Defense Secretary Lloyd Austin. “We will continue to make clear to the Houthis that they will bear the consequences if they do not stop their illegal attacks.”

The Houthis denounced the “US-British aggression” and vowed to keep up its military operation in response. “The Yemeni Armed Forces affirm that they will confront the US-British escalation with more qualitative military operations against all hostile targets in the Red and Arabian Seas in defense of our country, our people and our nation,” it said in a statement.

The U.S., U.K., and other allies said in a statement the “necessary and proportionate strikes specifically targeted 18 Houthi targets across 8 locations in Yemen” that also included underground storage facilities, radar and a helicopter.

U.K. Defense Secretary Grant Shapps said RAF Typhoon jets engaged in “precision strikes” aimed at degrading Houthi drones and launchers. Shapps said it came after “severe Houthi attacks against commercial ships in the Red Sea and Gulf of Aden, including against the British-owned MV Islander and the MV Rubymar, which forced the crew to abandon ship.” It’s the fourth time Britain has joined in the U.S.-led strikes.

The strikes have support from the wider coalition, which includes Australia, Bahrain, Canada, Denmark, the Netherlands and New Zealand.

President Joe Biden and other senior leaders have repeatedly warned that the U.S. won’t tolerate the Houthi attacks against commercial shipping. But the counterattacks haven’t appeared to diminish the Houthis’ campaign against shipping in the region, which the militants say is over Israel’s war against Hamas in the Gaza Strip.

“Our aim remains to de-escalate tensions and restore stability in the Red Sea, but we will once again reiterate our warning to Houthi leadership: we will not hesitate to continue to defend lives and the free flow of commerce in the face of continued threats,” said the Saturday statement.

The Houthis have launched at least 57 attacks on commercial and military ships in the the Red Sea and Gulf of Aden since Nov. 19, and the pace has picked up in recent days.

“We’ve certainly seen in the past 48, 72 hours an increase in attacks from the Houthis,” Pentagon spokeswoman Sabrina Singh said in a briefing Thursday. And she acknowledged that the Houthis have not been deterred.

“We never said we’ve wiped off the map all of their capabilities,” she told reporters. “We know that the Houthis maintain a large arsenal. They are very capable. They have sophisticated weapons, and that’s because they continue to get them from Iran.”

There have been at least 32 U.S. strikes in Yemen over the past month and a half; a few were conducted with allied involvement. In addition, U.S. warships have taken out dozens of incoming missiles, rockets and drones targeting commercial and other Navy vessels.

Earlier Saturday, the destroyer USS Mason downed an anti-ship ballistic missile launched from Houthi-held areas in Yemen toward the Gulf of Aden, U.S. Central Command said, adding that the missile was likely targeting MV Torm Thor, a U.S.-Flagged, owned, and operated chemical and oil tanker.

The U.S. attacks on the Houthis have targeted more than 120 launchers, more than 10 surface-to-air-missiles, 40 storage and support building, 15 drone storage building, more than 20 unmanned air, surface and underwater vehicles, several underground storage areas and a few other facilities.

The rebels’ supreme leader, Abdul Malik al-Houthi, announced this past week an “escalation in sea operations” conducted by his forces as part of what they describe as a pressure campaign to end Israel’s war on Hamas.

But while the group says the attacks are aimed at stopping that war, the Houthis' targets have grown more random, endangering a vital waterway for cargo and energy shipments traveling from Asia and the Middle East onward to Europe.

During normal operations, about 400 commercial vessels transit the southern Red Sea at any given time. While the Houthi attacks have only actually struck a small number of vessels, the persistent targeting and near misses that have been shot down by the U.S. and allies have prompted shipping companies to reroute their vessels from the Red Sea.

Instead, they have sent them around Africa through the Cape of Good Hope — a much longer, costlier and less efficient passage. The threats also have led the U.S. and its allies to set up a joint mission where warships from participating nations provide a protective umbrella of air defense for ships as they travel between the Suez Canal and the Bab el-Mandeb Strait.

In Thursday’s attack in the Gulf of Aden, the Houthis fired two missiles at a Palau-flagged cargo ship named Islander, according to Central Command said. A European naval force in the region said the attack sparked a fire and wounded a sailor on board the vessel, though the ship continued on its way.

Central Command launched attacks on Houthi-held areas in Yemen on Friday, destroying seven mobile anti-ship cruise missiles that the military said were prepared to launch toward the Red Sea.

Central Command also said Saturday that a Houthi attack on a Belize-flagged ship on Feb. 18 caused an 18-mile (29-kilometer) oil slick and the. military warned of the danger of a spill from the vessel’s cargo of fertilizer. The Rubymar, a British-registered, Lebanese-operated cargo vessel, was attacked while sailing through the Bab el-Mandeb Strait that connects the Red Sea and the Gulf of Aden.

The missile attack forced the crew to abandon the vessel, which had been on its way to Bulgaria after leaving Khorfakkan in the United Arab Emirates. It was transporting more than 41,000 tons of fertilizer, according to a Central Command statement.

The Associated Press, relying on satellite images from Planet Labs PBC of the stricken vessel, reported Tuesday that the vessel was leaking oil in the Red Sea.

Yemen’s internationally recognized government on Saturday called for other countries and maritime-protection organizations to quickly address the oil slick and avert “a significant environmental disaster.

Sinking ship hit by Houthi missile leaves 18-mile oil slick in Red Sea, US officials say.

A cargo ship that was struck by a Houthi ballistic missile on Monday has created an 18-mile long oil slick in the Red Sea as it continues to take on water, two US officials said Friday.

The M/V Rubymar — a Belize-flagged, UK-registered, Lebanese-owned vessel — was carrying 41,000 tons of fertilizer when it was struck on Monday by one of two ballistic missiles fired from Houthi territory in Yemen.

US Central Command said the ship is currently anchored as it takes on water. “The Houthis continue to demonstrate disregard for the regional impact of their indiscriminate attacks, threatening the fishing industry, coastal communities, and imports of food supplies,” US Central Command said.

One of the US officials said the threat of more Houthi attacks in the Red Sea, combined with the condition of the water, makes it incredibly difficult to safely get to the ship and attempt to tow it to a port.

The damage sustained by the Rubymar is potentially the most significant to a vessel caused by an attack launched by the Houthis, who have been targeting commercial shipping in the Red Sea and Gulf of Aden for months.

It remains unclear what kind of substance is causing the slick.

US Central Command said this week that the Rubymar sent out a distress call after the attack and was assisted by a coalition warship and another merchant vessel, which took the crew to a nearby port. It appeared to be the first time a crew has been forced to evacuate a ship after it was hit by the Houthis. Many of the ships struck by Houthi missiles have been able to continue their voyage.

Deputy Pentagon Press Secretary Sabrina Singh said Thursday that the Rubymar was “taking on water as we speak.”

“It’s creating an environmental hazard with the leakage of all the fuel that it’s carrying,” Singh said. “On top of that, it was carrying, to my understanding, fertilizer. So the Houthis are creating an environmental hazard right in their own backyard.”

CNN reported on Friday that the Biden administration is struggling to stop the ongoing attacks by the Houthis against ships in the Red Sea, and the group is continuing to fortify its weapons stockpile inside Yemen, even though the US has carried out significant strikes on the group in recent weeks.

The Houthis’ attacks have been ongoing for months, and despite several rounds of strikes by the US and UK on their capabilities, US officials told CNN it’s unclear how much weaponry the militia group still has.

“The US campaign against the Houthis appears to bear the hallmarks of many of these highly circumscribed, scrubbed campaigns of the past where we seek to avoid causing them actual pain,” a former US military official told CNN.

The Houthis’ attacks have increased in recent days; Singh said Thursday there has “certainly” been “an increase in attacks from the Houthis” over the last 72 hours. And while the Houthis have said they are conducting the attacks in support of the Palestinian people and targeting ships connected to Israel, many of the vessels attacked have instead been connected to other countries.

One other ship hit by the Houthis on Monday — the M/V Sea Champion, a US-owned, Greek-flagged bulk carrier — was carrying grain to Yemen. A CENTCOM release on the attack said the Sea Champion has “delivered humanitarian aid to Yemen 11 times in the past five years.”

“So, again, they’re saying that they’re conducting these attacks against ships that are connected to Israel,” Singh said Thursday. “These are ships that are literally bringing goods, services, aid to their own people, and they’re creating their own international problem.”

Houthi attacks on ships are giving the rebels and Iran intel on how to hunt down and strike naval targets, war experts say.

• The Houthis have spent the past few months lobbing missiles and drones at ships off Yemen's coast.

• These threats have not hit US Navy ships in the region, but they have struck merchant vessels.

• War experts say these attacks are giving Iran and the rebels, Tehran's proxies, valuable data.

After months of constantly firing missiles and drones at ships off the coast of Yemen, the Houthis are showing no signs of ending their relentless provocations.

During this time, their weapons have struck several commercial vessels, come dangerously close to a US Navy warship, and introduced a deadly new threat into naval combat. War experts say the rebels and Iran, their main backer, have been learning key information from the ongoing attacks in the Red Sea and Gulf of Aden.

Analysts at the Institute for the Study of War and American Enterprise Institute think tanks said the Houthis and Iran are likely using the attacks to "test and refine their approach to striking naval targets." Tehran's military advisors have reportedly been providing the rebels with critical targeting intelligence.

"These Houthi attacks provide Iran and the Houthis opportunities to evaluate the effectiveness of different strike packages to understand how they can evade and overwhelm US defenses more effectively," these analysts wrote in a Thursday assessment of conflicts across the Middle East.

The Houthi rebels boast a sizable arsenal of one-way attack drones, anti-ship cruise missiles, and anti-ship ballistic missiles, the latter of which had never been used in combat until recently. They also possess an inventory of surface and underwater drones.

Some of the capabilities, like the anti-ship missiles, are Iranian in origin or are made up of parts from the country, according to an analysis the International Institute for Strategic Studies think tank published in early January.

As the Houthis continue to lose missiles and drones to US military intercepts and strikes — preemptive actions designed to take out the threats before they launch — it is unclear how often the rebels are being rearmed and resupplied by Iran.

American forces have, on several occasions over the past few weeks, intercepted small boats smuggling advanced weaponry from Tehran to Yemen in a bid to stem the flow of arms to the Houthis.

"We still believe that they continue to be supported by the regime in Tehran — materials, weapons systems are still being supplied," White House National Security Council spokesperson John Kirby told reporters on Friday, adding that the US is continuing to conduct operations to intercept Houthi-bound weaponry at sea.

"We'll obviously do everything we can to try to limit and stem that flow, but the flow is ongoing," he said.

But despite losing combat capabilities to US forces, the Houthis still manage to consistently launch threats into key international shipping lanes off the coast of Yemen. And while the rebels have been unable to score a hit on the incredibly well-defended American warships operating in the region, they have struck multiple commercial vessels with drones and missiles.

These hits have largely caused only minor damage to the ships, and they were still able to keep sailing toward their destinations. On occasion though, these engagements prove to be more costly. Earlier this week, for example, a Houthi anti-ship ballistic missile hit a bulk carrier, forcing its crew to issue a distress call and abandon the vessel.

The frequency of the Houthi missile and drone attacks has raised questions about how long these engagements might go on for and the sustainability of the US Navy's operations in the region.

US officials continue to assert that the military's strikes against the Houthis — some of which have been widespread and coordinated with the UK while the vast majority are unilateral — have managed to degrade the rebels' capabilities to a notable extent.

Sabrina Singh, the deputy Pentagon press secretary, told reporters this week that the Houthis still have a large number of capabilities, but US and coalition forces have been able to chip away at this. And the White House has said the same.

"We do believe that we have had an impact on not just the degradation of their capabilities, but the way in which they're using the capabilities they have available to them," Kirby said on Friday.