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

between China and its neighbors will be in the spotlight on

Thursday as leaders of the U.S., Japan and the Philippines

gather 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 will announce new joint military

efforts and infrastructure spending in the former American

colony as he hosts Philippines President Ferdinand Marcos Jr.

and 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 cannon 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.

Biden will affirm that a 1950s era mutual defense treaty

binding Washington and Manila would require the United States to

respond to an armed attack on the Philippines in the Second

Thomas Shoal, one of the U.S. officials said.

Marcos has successfully pushed Washington to resolve

longstanding ambiguity over the treaty by specifying that it

would apply to disputes in that sea.

"What you'll see is a clear demonstration of support and

resolve from both President Biden and Prime Minister Kishida

that we stand shoulder to shoulder with Marcos, ready to support

and work with the Philippines at every turn," the official said.

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.

"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 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, an 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.

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.