A dramatic stand-off with Beijing in the South China Sea this week was the most serious incident yet for the Philippines, its top security officials said on Wednesday, vowing not to back down in asserting the country's sovereign rights.

A Chinese Coast Guard vessel blocks the Philippine resupply vessel Unaizah May 4, on its way to a resupply mission at Second Thomas Shoal in the South China Sea.

The Philippines has been incensed by what it calls repeated aggressive conduct by China's coastguard, accusing its ships of using water cannon and blocking and harassing a Philippine resupply mission on Tuesday for troops stationed at the disputed Second Thomas Shoal.

The Philippines' South China Sea task force said a top admiral was on board a vessel that was water cannoned by China's coastguard, shattering its windshield and wounding four navy personnel. The admiral was unharmed.

"This is the most serious incident yet," task force spokesperson Jonathan Malaya said, accusing China of "deliberately stirring up trouble" and "maliciously inciting hype".

China accused the Philippines of intruding on its territory, claiming indisputable sovereignty over the reef, located 1,300 km (808 miles) off its mainland. China claims most of the South China Sea as its own, despite an international arbitration panel concluding that position had no basis under international law.

Philippine Defense Secretary Gilberto Teodoro on Wednesday said China's claims were baseless and its actions this week were "patently illegal and downright uncivilised".

"This claim is, simply put, one that no right-thinking state in the world agrees with and which many outright condemn," Teodoro said in a statement.

"(Its) vain attempt to manufacture and sell this story falters in the face of real incontrovertible facts."

INCREASED FLARE-UPS

Tuesday's incident was the latest in a series of run-ins between the Philippines and China over disputed areas of the South China Sea, coinciding with a recent surge in defence activities between the militaries of Manila and Washington.

Australia and Southeast Asian nations on Wednesday called for restraint in the contested South China Sea and adherence to a "rules-based" order in the Indo-Pacific.

"We encourage all countries to avoid any unilateral actions that endanger peace, security and stability in the region," they said in a joint statement after a three-day meeting.

A spokesperson for the Chinese foreign ministry said the situation was generally stable and China's position on the Second Thomas Shoal was consistent and clear.

The U.S. State Department said it stands by the Philippines following "provocative actions" by China.

The Philippines and United States have a Mutual Defence Treaty binding them to defend each other if attacked, raising the stakes in a region where tensions have simmered for decades over Beijing's claims to territory in the South China Sea, a key conduit for global commerce.

Philippine officials on Wednesday said invoking that pact would be a serious matter, although consultations were taking place between both countries.

Speaking while in Australia, however, Philippine President Ferdinand Marcos Jr said it was not the time nor reason to invoke the treaty, but the incident was being viewed "with great alarm".

The Philippine foreign ministry said its embassy in China has issued a "demarche" or formal reprimand to its Beijing counterpart.

Philippines says crew hurt, vessels damaged by China Coast Guard

A frame grab from aerial video footage shows the collision between ships of the Chinese and Philippine coast guards.

The Philippines said Tuesday that China Coast Guard vessels caused two collisions with Philippine boats and water cannoned one of them, leaving four crew injured during a resupply mission in the South China Sea.

Manila summoned a Chinese representative over the incidents, which happened in waters around Second Thomas Shoal in the Spratly Islands where the countries have contesting maritime claims.

A Philippine government task force said in a statement that "China Coast Guard and Chinese Maritime Militia vessels harassed, blocked, deployed water cannons, and executed dangerous maneuvers in another attempt to illegally impede or obstruct a routine resupply and rotation mission".

The China Coast Guard said it "took control measures" against Philippine ships' "illegal intrusion" in waters around the shoal.

The BRP Sindangan, along with a sister ship, had been deployed to support the military chartered Unaizah May 4 and Unaizah May 1 boats carrying replacement soldiers and supplies to Second Thomas Shoal, where Filipino troops are stationed on a grounded Philippine navy vessel, the BRP Sierra Madre.

Four crew on board the Unaizah May 4 were injured when two China Coast Guard ships simultaneously fired water cannon at it, shattering the windshield, the National Task Force for the West Philippine Sea said in a statement.

It said a China Coast Guard vessel also caused a "minor collision" with the Unaizah May 4, which turned back to shore without delivering its cargo.

The Unaizah May 1 was able to unload its provisions, which the troops on the rusty BRP Sierra Madre depend on for their survival.

Earlier in the morning, the task force added, a China Coast Guard vessel caused a separate "minor collision" with one of the Philippine Coast Guard ships that was escorting the supply boats.

China Coast Guard spokesman Gan Yu accused the Philippine Coast Guard ship of "intentionally" ramming the Chinese boat, causing a "minor scrape".

Chinese foreign ministry spokeswoman Mao Ning said Beijing had expressed "strong protests" over the run-in with the Philippine vessels which were "attempting to transport construction supplies and building materials" to the Sierra Madre.

The Philippine Department of Foreign Affairs said it told Chinese embassy deputy chief of mission Zhou Zhiyong that the "aggressive actions" of the China Coast Guard and other vessels were "unacceptable".

It ordered the Chinese vessels to leave the vicinity of the reef "immediately", the department said in a statement.

China claims almost the entire South China Sea, brushing aside competing claims from a host of Southeast Asian nations and an international ruling that has declared its stance baseless.

Second Thomas Shoal is about 200 kilometres (124 miles) from the western Philippine island of Palawan, and more than 1,000 kilometres from China's nearest major landmass, Hainan island.

- 'Stop harassing us' -

The incident comes a day after Philippine Foreign Minister Enrique Manalo called on China to "stop harassing us" as he defended Manila's strategy of publicising Chinese manoeuvres in the South China Sea.

Philippine President Ferdinand Marcos was similarly forthright when he appeared later Monday evening at an event hosted by an Australian think tank.

"We shall never surrender even a square inch of our territory and our maritime jurisdiction," he said on the sidelines of the ASEAN summit in Melbourne.

Tuesday's collisions and water cannoning came after similar incidents in December. Those confrontations were the most intense between Philippine and Chinese vessels in years.

Beijing is trying to "break our resolve, basically warn us that this will lead to further escalation," political science professor Renato de Castro of Manila's De La Salle University told AFP.

Relations between Manila and Beijing have frayed under Marcos, who has sought to improve ties with traditional ally Washington and deepen defence cooperation in the region, while also pushing back against Chinese actions in the South China Sea.

That contrasts with the approach of former president Rodrigo Duterte, who set aside maritime disputes with Beijing in exchange for promises of Chinese investment.

US ambassador to the Philippines MaryKay Carlson condemned China's "dangerous maneuvers" in a post on social media platform X, saying they "endangered lives, resulted in injuries, and damaged (Philippine Coast Guard) vessels".

Luc Veron, the EU envoy to Manila, said on X he was "troubled" by China's efforts to target "Philippine vessels engaged in crucial resupply missions".

Political analyst Richard Heydarian said the Chinese actions could lead "to a horrible accident or an extremely violent incident that could really escalate out of control in ways that is not only detrimental to the Philippines but also to the supposedly stronger party."