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.

They 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.

The funding is part of the $2 billion in foreign military financing approved by the United States in April. It comes as the Philippine modernises its armed forces, one of the weakest in Asia, and bolsters its coast guard.

The Philippines' proximity to the hotly contested South China Sea, as well as self-ruled Taiwan, would make it a key partner for the United States if a conflict were to break out in the region.