Manila's resupply missions to troops on a disputed South China Sea atoll will remain purely Philippine operations, the National Security Council said Sunday, after Washington vowed to "do what is necessary" to support them.

The Second Thomas Shoal, which hosts a tiny Philippine garrison stationed on a deliberately beached old warship, has been a focus of escalating confrontations between Chinese and Philippine ships in recent months as Beijing steps up efforts to push its claims to the South China Sea.

A Filipino sailor lost a thumb on the latest 17 June clash when Chinese coast guard members wielding knives, sticks and an axe foiled a Philippine Navy attempt to resupply its troops.

"As far as the RORE (rotation and resupply mission) is concerned, we're keeping it as a purely Philippine operation utilizing Philippine ships, personnel and leadership," National Security Council spokesman assistant director general Jonathan Malaya told AFP.

"That may change depending on the guidance from top management but that's the direction or policy at present."

Malaya's remarks came after White House National Security Adviser Jake Sullivan said the United States "will do what is necessary" to ensure the Philippines can continue to resupply its troops on the contested atoll.

"We will continue to support the Philippines and stand behind them as they take steps to be able to ensure that," Sullivan said during the Aspen Security Forum conference in Colorado.

Malaya said the National Security Council appreciated the US offer and the Philippines would continue consultations as treaty allies.

Manila has a mutual defence pact with the United States which 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.

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 kilometres from China's nearest major landmass, Hainan island.

Filipino soldiers stationed on the shoal live on the crumbling BRP Sierra Madre and require frequent resupplies for food, water and other necessities as well as transport for personnel rotations.

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