The United States will provide $500 million in military funding to the Philippines, US Secretary of State Antony Blinken said Tuesday, as Washington boosts ties with Manila in the face of China's growing assertiveness.

Blinken was in Manila with US Defense Secretary Lloyd Austin as part of an Asia-Pacific tour to strengthen Washington's latticework of alliances aimed at countering Beijing.

"We're now allocating an additional $500 million in foreign military financing to the Philippines to boost security collaboration with our oldest treaty ally in this region," Blinken told a joint news conference.

Blinken described it as a "once in a generation investment" to help modernise the Philippine armed forces and coast guard.

"We're building on a lot of progress the Philippines has already made to be better positioned to defend their sovereignty. That is what this is about."

Blinken and Austin earlier met with President Ferdinand Marcos, who has taken a strong stand against Chinese actions in the South China Sea, before holding "2+2" talks with their Philippine counterparts Enrique Manalo and Gilberto Teodoro.

The latest high-level US visit follows a series of escalating confrontations between Philippine and Chinese vessels in the disputed waterway that have raised concern that Washington could be dragged into a conflict due to its mutual defence treaty with Manila