A South China Sea confrontation this week, in which Chinese coast guard personnel brandished weapons and rammed Philippine naval boats, does not trigger Manila's mutual defense pact with the United States, a presidential spokesperson said Friday.

Footage released by Manila showed Chinese coast guard sailors brandishing knives, an axe, and other weapons in Monday's clash, in which a Filipino sailor lost a thumb, as they stopped a Philippine navy attempt to resupply a Filipino garrison on a derelict warship.

"We are not yet ready to consider this as an armed attack," President Ferdinand Marcos's executive secretary Lucas Bersamin told reporters when asked if Manila would ask Washington to honour the 1951 treaty.

The clash was the latest in a series of escalating confrontations as Beijing steps up efforts to push its claims to the disputed area.

The US-Philippines mutual defence pact requires both parties to come to the other's defence in case of an "armed attack" against vessels, aircraft, military and coast guard anywhere in the Pacific theatre, which Washington says includes the South China Sea.

US Secretary of State Antony Blinken spoke with Philippine Foreign Secretary Enrique Manalo on Wednesday about the recent "escalatory actions" by China, the State Department said in a statement.

Blinken said China's actions "undermine regional peace and stability and underscored the United States' ironclad commitments to the Philippines under our Mutual Defense Treaty".

Equipment destroyed

The Filipino sailor lost a thumb when a small Chinese boat rammed an inflatable boat from the Philippine navy.

The Chinese coast guard also confiscated or destroyed Philippine equipment, including guns and inflatable boats, the Philippine military said.

Beijing insisted its coast guard behaved in a "professional and restrained" way and blamed Manila for the confrontation, alleging the Philipines had been trying "to sneak in building materials, but also tried to smuggle in military equipment" to the remote garrison.

The Sierra Madre warship was deliberately grounded atop Second Thomas Shoal in 1999 to assert Manila's territorial claims.

Beijing claims almost the entirety of the South China Sea, brushing aside competing claims from several Southeast Asian nations including the Philippines and an international ruling that its stance has no legal basis.

Second Thomas Shoal lies about 200 kilometers (120 miles) from the western Philippine island of Palawan and more than 1,000 kilometers from China's nearest major landmass, Hainan island.

China deploys coast guard and other boats to patrol the waters around the shoal and has turned several reefs into artificial militarised islands.

Monday's clash has heightened fears that China would attempt to board the Sierra Madre and force out the Filipino troops manning it.

"That is always a consideration, and we will be prepared for that," Andres Centino, a retired Philippine general who is now a presidential adviser on maritime concerns, told reporters Friday.

However, Bersamin, Marcos's executive secretary, appeared to downplay the clash.

"This was probably a misunderstanding or an accident," he said, adding that the Chinese side had not used anything "beyond" bladed weapons.

"I think this is a matter that can easily be solved by us very soon, and if China wants to work with us we can work with China."

He said the government's advisory National Maritime Council had convened after the confrontation and recommended to the president that Manila publish the schedule of its resupply missions to the Sierra Madre in advance in the hope that China would act in a more restrained manner. 

Manila would not be "giving up anything" by adopting such a stance, he said.

Marcos has yet to approve the proposal, Bersamin and Centino said.