WASHINGTON, April 11 (Reuters) - Long-simmering tensions

between China and its neighbors took center stage on Thursday as

leaders of the U.S., Japan and the Philippines gathered at the

White House to push back on Beijing's stepped-up pressure on

Manila in the disputed South China Sea.

U.S. 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 center 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 defense treaty binding

Washington and Manila would require the U.S. to respond to an

armed attack on the Philippines in the South China Sea.

"United States defense 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 neighboring nations. The Second

Thomas Shoal is within the Philippines' 200-mile (320-km)

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

criticized both the U.S. and Japan for ratcheting up tensions.

The U.S. 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 U.S.

official said.

The moves come after two prominent U.S. senators on

Wednesday introduced a bipartisan bill to provide Manila with

$2.5 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 U.S. 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 defense 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.

Biden's Partnership for Global Infrastructure and Investment

will back a new Luzon corridor effort in the Philippines, aimed

at infrastructure projects including ports, rail, clean energy

and semiconductor supply chains.