President Biden intends to use a first-ever joint meeting with the leaders of Japan and the Philippines on Thursday to send a blunt diplomatic message to an increasingly aggressive China: Beijing’s harassment of Philippine ships in the South China Sea is a violation of international law and must stop.

In recent months, Chinese coast guard ships have been ramming Philippine vessels, blasting them with water cannons and aiming lasers at their crews in what the United States condemns as “coercive and unlawful tactics” in one of the most crucial waterways in the world.

So far, the Chinese provocations, asserting disputed claims to the international waters, have fallen short of the kinds of attacks that would trigger the military defense pact that the United States and the Philippines signed in 1951. But Biden administration officials said the meeting of the three leaders on Thursday is intended to demonstrate to China even stronger military and diplomatic unity among the leaders of the three allies.

One U.S. official, speaking on the condition of anonymity to discuss the meeting in advance, called the issue of security in the South China Sea a “pillar” of the discussions between Mr. Biden, Prime Minister Fumio Kishida of Japan and President Ferdinand Marcos Jr. of the Philippines.

“The U.S., Japan, and the Philippines are three closely aligned maritime democracies with increasingly convergent strategic objectives and interests,” Jake Sullivan, the president’s national security adviser, said on Tuesday. “Just this past week, our three countries and Australia held joint naval drills in the South China Sea.”

Officials said there would be similar drills in the months ahead as the nations continue to assert the freedom of travel through international waters that China claims as its own. They called Thursday’s meeting at the White House a demonstration of support from Mr. Biden and Mr. Kishida for the Philippines in its clashes with China.

China has asserted greater control over the South China Sea over the years, trying to expand its military footprint in the region.

The meeting of the three leaders comes a day after Mr. Biden hosted Mr. Kishida at the White House for meetings and a state dinner. The two men discussed China’s military and economic aggression but also announced a series of new initiatives to foster more cooperation on the economy, space exploration, technology and research.

Officials said a similar list of announcements was expected to follow Thursday’s meeting between the three leaders.

The leaders were set to announce new investments in infrastructure projects in the Philippines aimed at improving what they called “high-impact” project such as ports, rail, clean energy and semiconductor supply chains.

They were also expected to unveil new efforts by the United States and Japan to install radio access network technology in the Philippines, a modernization effort that will improve the ability of people to communicate wirelessly throughout the region, officials said.

Officials said there would also be other announcements about collaboration among the three nations on global humanitarian assistance efforts and on even greater cooperation among the militaries of the three countries.