US President Joe Biden's administration is expected to announce new joint military efforts and infrastructure spending in the former American colony while he hosts Philippines President Ferdinand Marcos Jr. alongside Japanese Prime Minister Fumio Kishida in Washington for a first-of-its-kind trilateral summit.
Topping the meeting's agenda is China's increasing pressure in the South China Sea, which has escalated despite a personal appeal by Biden to Chinese President Xi Jinping last year.
The Philippines and China had several maritime run-ins last month that included the use of water cannons and heated verbal exchanges. The disputes centre on the Second Thomas Shoal, home to a small number of Filipino troops stationed on a warship that Manila grounded there in 1999 to reinforce its sovereignty claims.
Launching the White House meeting with the three leaders, Biden affirmed that a 1950s era mutual defence treaty binding Washington and Manila would require the US to respond to an armed attack on the Philippines in the South China Sea.
"United States defence commitments to Japan and to the Philippines are iron clad," he said.
Marcos has successfully pushed Washington to resolve longstanding ambiguity over the treaty by specifying that it would apply to disputes in that sea.
China claims almost the entire South China Sea, including the maritime economic zones of neighbouring nations. The Second Thomas Shoal is within the Philippines' 320km exclusive economic zone. A 2016 ruling by the Permanent Court of Arbitration found that China's sweeping claims have no legal basis.
Japan has a dispute with China over islands in the East China Sea.
Chinese foreign ministry spokesperson Mao Ning said "China's activities in the East China Sea and South China Sea fully comply with international law" and that they were prepared to resolve issues through "dialogue and consultation" but criticised both the US and Japan for ratcheting up tensions.
The US plans a Coast Guard joint patrol in the Indo-Pacific region in the coming year as well as joint maritime training activities. Washington will also place "humanitarian relief commodities for Philippine civilian disaster response" at Philippine military bases, a senior Biden administration official said.
More joint patrols in the coming months can be expected in the South China Sea after drills by the United States, Australia, the Philippines and Japan last weekend, another US official said.
The moves come after two prominent US senators on Wednesday introduced a bipartisan bill to provide Manila with $US2.5 billion to boost its defenses against Chinese pressure.
"China's frequent tactic is to try to isolate the target of its pressure campaigns, but the April 11 trilateral signals clearly that the Philippines is not alone," said Daniel Russel, who served as the top US diplomat for East Asia under former president Barack Obama.
The leaders will also discuss broader regional challenges and economic development, with new investments coming in undersea cables, logistics, clean energy and telecommunications. Facebook parent Meta and UPS are among the companies announcing deals related to the visit.
A new air missile defence network announced on Wednesday involving Japan and Australia and focused on the Indo-Pacific region is still "probably a few years off," one of the officials said.
