Monday's incident was the latest in a series of escalating confrontations between Chinese and Philippine ships in recent months as Beijing steps up efforts to push its claims to the disputed area.

Filipino military chief General Romeo Brawner alleged the Chinese boarders were armed with swords, spears, and knives in the incident off Second Thomas Shoal.

He denounced the Chinese action as "piracy" and demanded the return of seized weapons and other equipment, as well as reparations for damaged items.

China defended its actions, with its foreign ministry saying on Wednesday that "no direct measures" were taken against Filipino personnel.

Images from China's Coast Guard shared by state media purported to show the confrontation on Monday, declaring: "The Chinese coast guard carried out boarding and inspection of a Philippine boat for the first time".

Photos showed two Chinese vessels approaching what it said was a smaller Philippine vessel from either side while another Chinese boat tailed closely behind.

They showed Chinese coast guard officials then boarding the encircled vessel.

AFP was not able to independently verify the images.

"Law enforcement measures taken by the China Coast Guard at the site were professional and restrained," foreign ministry spokesman Lin Jian told a regular news briefing in Beijing.

Second Thomas Shoal hosts a tiny Philippine garrison stationed on a deliberately beached old warship and has been the focus of a series of recent confrontations.

Top US diplomat Antony Blinken spoke with his Filipino counterpart 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."

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 kilometres (120 miles) from the western Philippine island of Palawan and more than 1,000 kilometres from China's nearest major landmass, Hainan island.

- 'Bare hands' -

Brawner described the incident after visiting the injured sailor in Palawan, saying Filipino personnel had "fought back with our bare hands" after Chinese coast guard personnel from eight vessels boarded their rigid-hulled inflatable boats.

"They took guns and other equipment, destroyed our equipment on board, including the motors. They punctured our rigid-hulled inflatable boats," he said.

"We are demanding that the Chinese return our rifles and our equipment and we are also demanding from them to pay for the damages that they have caused," Brawner said.

"For me this is piracy."

Rear Admiral Alfonso Torres, commander for the Philippines' South China Sea forces, told reporters "seven firearms were looted and forcibly taken" by the Chinese coast guard.

He said communication and navigation equipment and an outboard motor were destroyed and the mobile phones of the Filipino crew were taken.

Brawner said the Chinese coast guard carried "bolos" -- a type of single-edged sword -- as well as spears and knives.

"Our troops had none of those," he said.

The Philippine military said the seized guns were intended for Filipino troops manning the BRP Sierra Madre warship on the shoal. The crew of the boarded boats were under orders not to display their weapons, it said.

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

It has stepped up moves against Philippine vessels in the area in recent months.

Torres said Monday's confrontation began when one of the Filipino boats was "illegally rammed" at "high speed" by a Chinese coast guard boat.

The injured Filipino Naval Special Operations Group member lost a thumb when the Chinese vessel landed on top of the bow, Torres said.

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ADMIRAL GROUP