In a warning to China, Biden is hosting a summit with the leaders of Japan and the Philippines

President Biden used the first-ever joint meeting with the leaders of Japan and the Philippines on Thursday to expand the network of security and economic alliances in the Indo-Pacific region that US officials believe will serve as a shield against Chinese aggression.

Flanked by his counterparts and senior diplomatic aides at the White House, Biden said the two countries were “deepening our maritime and security ties” and delivered a blunt message clearly targeting China's actions in the South China Sea.

“I want to be clear, the United States’ defense commitments to Japan and the Philippines are strict,” Biden said.

Jake Sullivan, White House national security adviser, described the diplomatic efforts in Southeast Asia as one of a series of efforts by like-minded countries to respond to China's activities in trade, technology and military aggression.

The goal differs from that achieved in Europe, where countries came together after World War II in a single alliance known as NATO. Analysts say that instead of a single group, the United States and countries in the region are forming smaller, overlapping partnerships aimed at ensuring they can withstand Chinese pressure.

“China uses a very powerful combination of coercion and international trade, along with its growing naval power,” said Rana Mitter, a professor at Harvard's Kennedy School of Government. He said the United States, Japan and the Philippines are seeking to demonstrate that they have “an ecosystem of different allies that are trying to respond” to this kind of pressure.

This strategy came to light on Thursday when the three leaders stressed the need for unity, although none of them mentioned China by name.

Japanese Prime Minister Fumio Kishida declared that “multi-layered cooperation is essential” for the future of the region. Philippine President Ferdinand Marcos Jr. said the meeting highlights three countries “bound by a deep respect for democracy, good governance and the rule of law.”

“When we stand as one, we are able to forge a better peace for everyone,” Mr. Biden concluded.

Strengthening alliances is unlikely to provide a short-term solution to Beijing's harassment of Philippine ships in the South China Sea, which the United States and its allies have said is a violation of international law and must stop.

But Biden administration officials said the meeting of the three leaders showed China stronger military and diplomatic unity among the three allied leaders.

Chinese coast guard vessels are ramming Filipino ships, firing water cannons at them and targeting their crews with lasers in what the United States condemns as “coercive and illegal tactics” in one of the world’s most important waterways.

So far, Chinese provocations have fallen short of the attacks that would trigger the military defense agreement the United States and the Philippines signed in 1951.

One US official, who spoke on condition of anonymity to discuss the meeting before it took place, described the issue of security in the South China Sea as a “pillar” of the discussions.

“The United States, Japan and the Philippines are three closely aligned maritime democracies with increasingly converging strategic goals and interests,” Mr. Sullivan said on Tuesday. He added: “Just last week, our three countries and Australia conducted joint naval exercises in the South China Sea.”

Officials said there will be similar exercises in the coming months as countries continue to assert freedom of travel through international waters that China claims as its own.

Mr. Mitter said the prospect of future naval exercises — perhaps near the Philippines — would be one of the strongest messages the three countries could send. China has asserted greater control over the South China Sea over the years, in an attempt to expand its military presence in the region.

“I think they will take it seriously,” he said of the Chinese leadership, noting that a show of military unity might prompt the government there to reduce harassment in the short term.

But in the longer term, he added, Japan and the Philippines were increasingly keen to create a network of alliances with each other that could continue even if the United States scaled back its involvement under a more isolationist administration. If former President Donald J. Trump won a second term.

“It could be very difficult,” he said of a possible victory for Mr. Trump. “America’s allies in the region are very keen for the United States to remain in the region and have a presence there.”

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

Officials from the three countries issued a similar list of announcements after Thursday's meeting.

Officials said the countries will make new investments in infrastructure projects in the Philippines with the aim of improving what they called “high-impact” projects such as ports, railways, clean energy and semiconductor supply chains.

They also revealed new efforts by the United States and Japan to install wireless access network technology in the Philippines, an upgrade aimed at improving wireless communications throughout the region.

Officials also promised new cooperation between the three countries in global humanitarian aid efforts and even greater cooperation between the countries' militaries.