An escalating diplomatic row and recent maritime run-ins between China and the Philippines, a U.S. treaty ally, have made the highly strategic South China Sea a potential flashpoint between Washington and Beijing.

Philippine Coast Guard personnel prepare rubber fenders after Chinese Coast Guard vessels blocked their way to a resupply mission at the Second Thomas Shoal in the South China.

The issue will be a focus of trilateral meeting between U.S. President Joe Biden, Philippines President Ferdinand Marcos Jr. and Japan Prime Minister Fumio Kishida in Washington on Thursday.

WHAT ARE THE FLASHPOINTS?

Central to recent standoffs between the Philippines and China are two hotly contested features located inside Manila's 200-nautical mile exclusive economic zone, but which Beijing claims as its own.

China uses the so-called nine-dash line that takes in about 90% of the South China Sea to assert its claim to sovereignty over the Scarborough Shoal, a submerged reef coveted for its bountiful fish stocks, and the Second Thomas Shoal, home to a small contingent of Filipino sailors living aboard a rusty warship that Manila intentionally grounded in 1999 to further its territorial claims.

WHY ARE THINGS HEATING UP?

The Permanent Court of Arbitration in the Hague ruled in 2016 that Beijing's expansive claims via its nine-dash line had no basis under international law, handing the Philippines a landmark victory, but that has not stopped China, which rejects the ruling, from being more assertive.

Beijing has deployed hundreds of coastguard vessels to patrol those areas, alarming the Philippines, rival claimants and other states operating in the South China Sea, including the United States, which is wary about China's growing military power and territorial ambition.

WHAT HAVE THE STANDOFFS ENTAILED?

Encounters between the Philippines and China in Asia's most contested waters have grown tenser and more frequent over the past year as Beijing presses its claims and Manila refuses to cease its fishing and resupply activities to Filipinos at the two shoals. China considers those to be illegal intrusions and has tried to repel the vessels.

China's coastguard has stepped up so-called "grey-zone" activities such as use of water cannon, collision and ramming tactics, and, according to Manila, use of a military-grade laser, to try to stop the Philippine resupply and patrol missions. It has also deployed an armada of fishing boats the Philippines and allies consider militia.

During the last two Second Thomas Shoal resupply missions, Philippine boats sustained damage and some crew were injured after use of water cannon. China has urged the Philippines to tow away the warship, saying it had promised to do just that, but Manila maintains no such agreement was made.

WHAT WAS THE GLOBAL REACTION?

China's actions have drawn international condemnation and concern from major powers including the United States, Japan, Australia, France, and Britain.

Foreign journalists have joined some of the Philippine resupply missions and documented events at the invitation of the Philippines, which one security official said was aimed at "shedding light on China's 'grey zone' tactics". China has accused the Philippines of stirring up trouble and spreading misinformation.

China's actions are "dangerous, illegal and they are destabilising the region," a senior U.S. admiral said on April 9.

HOW IS THE PHILIPPINES RESPONDING?

President Ferdinand Marcos Jr has adopted a tough line against what he sees as Chinese hostility and rejected its pressure, recently vowing to implement countermeasures against "illegal, coercive, aggressive, and dangerous attacks" by China's coastguard, upping the ante in the escalating row.

The Philippines has said countermeasures will be "multi-dimensional" and involve exhausting diplomatic options. Marcos has also called for stronger coordination on maritime security to confront "a range of serious challenges" to territorial integrity and peace.

COULD THE UNITED STATES GET INVOLVED?

The Philippines' dispute with China coincides with an increase in security engagements with the United States under Marcos, including expansion of U.S. access to Philippine bases. Manila is also seeking close security ties with other allies like Japan and Australia. The engagements include joint patrols, which have frustrated China.

The United States has a Mutual Defense Treaty with the Philippines and has repeatedly made clear it would protect its ally if its coastguard or armed forces came under attack anywhere in the South China Sea, calling the agreement "ironclad".

The treaty raises the stakes significantly in the Philippines-China dispute in the event of a miscalculation at sea. However, it could also limit how far China is willing to go to keep the Philippines at bay, wary of the risks of conflict and pressure to respond resolutely if there were direct U.S. military involvement.

Philippine officials, including Marcos, have dismissed talk of invoking the treaty in the present situation, stressing it would be a last resort.

Expect more joint South China Sea patrols, US says ahead of summits with allies

Sullivan speaks at a press briefing at the White House in Washington

More joint patrols can be expected in the South China Sea after drills by the United States, Australia, the Philippines and Japan last weekend, U.S. national security adviser Jake Sullivan said on Tuesday, ahead of U.S. summits this week with the Japanese and Philippine leaders.

Warships from the four nations staged the exercises on Sunday following stepped up Chinese pressure on the Philippines in the disputed strategic waterway.

U.S. President Joe Biden hosts Japanese Prime Minister Fumio Kishida in Washington on Wednesday and the two and Philippines President Ferdinand Marcos will meet on Thursday for talks that will include ways to push back against China.

"On the naval patrols, we just saw trilateral plus Australia, a new form of quadrilateral joint naval patrols last week, so you can expect to see more of that in the future," Sullivan told a regular White House briefing while previewing the summits.

Sullivan also said Washington and its existing Australian and British partners in the AUKUS security pact would explore possible Japanese involvement in Pillar II of the project, something the Biden-Kishida summit would address.

"We're prepared to work with additional partners beyond the three of us, where they can bring capabilities, and Japan is one of the countries that could very well bring capabilities to that," Sullivan said.

"Japan could be a critical contributor," he said. "You will see when they speak tomorrow, an indication that that's the direction we're moving in."

Sullivan said Biden and Kishida would announce measures to enhance defense and security cooperation and in space exploration.

In an interview with Japan's NHK World Television conducted on Monday, Sullivan addressed plans for a new Japanese Self Defense Forces (SDF) headquarters overseeing all of the country's military operations that Kishida wants to establish by the end of March 2025.

Sullivan said Biden would give Kishida a "complete commitment ... to match Japan's upgrade of its own operational command, with an upgrade of our operational command."

Sources with knowledge of the planning have told Reuters Washington will consider appointing a four-star commander for Japan to match the rank of the head of Japan's new military headquarters. Experts say a U.S. officer of that rank could lay the groundwork for a future unified Japanese-U.S. command.

Sullivan said the specifics still had to be worked out, "but the overall strategic direction is clear." He said the aim would be to enable the two countries to "work together more effectively, jointly against all of the common challenges that we face in the security realm in the Indo-Pacific."

"We are prepared as Japan is to take a step forward in enhancing our operational command in Japan and in making sure that we're integrating our operational command with Japan's because we have to be able to work jointly in a world of dynamic threats and challenges in a region of dynamic threats and challenge."

He told NHK the space discussions would cover "our shared desire to return to the moon."

Japan is hoping to land its first astronaut on the moon with the U.S. Artemis project that envisages returning humans there by 2026, as competition with Russia and China intensifies.

Australia on Tuesday played down reports Japan could soon join AUKUS, saying any cooperation would be on a project-by-project basis as differences emerged within the pact over adding new members. A Japanese government official told Reuters on Monday discussions about formally joining AUKUS would likely not be welcomed by Australia or Britain until they had concrete results from the pact.

While Washington is keen on Japanese involvement, U.S. officials and experts say obstacles remain given the need for Tokyo to introduce better cyber defenses and stricter rules for guarding secrets.