President Biden and the leaders of Australia and the United Kingdom on Monday announced that Australia will purchase American nuclear-powered attack submarines to modernize its fleet amid growing concern about China’s influence in the Indo-Pacific.

Mr. Biden flew to San Diego to appear with the Australian and British prime ministers, Anthony Albanese and Rishi Sunak, as they hailed an 18-month-old nuclear partnership given the acronym Aukus — for Australia, the United Kingdom, and the United States. The partnership, announced in 2021, enables Australia to access nuclear-powered submarines, which are stealthier and more capable than conventionally powered vessels, as a counterweight to Communist China’s military expansion.

Mr. Biden, appearing sensitive to tensions with Beijing and its criticism of the deal, stressed that the submarines are “nuclear powered, not nuclear armed.”

“These boats will not have any nuclear weapons of any kind on them,” he said at an outdoor ceremony at Naval Base Point Loma in San Diego, where he was flanked by the British and Australian leaders. Two submarines, the USS Missouri and the USS Charlotte, were tied up at the next pier in the Pacific Ocean behind them.

Mr. Albanese said the agreement “represents the biggest single investment in Australia’s defense capability in all of our history.” It’s also the first time in 65 years that the U.S. has shared its nuclear propulsion technology, ”and we thank you for it,” he said.

Mr. Sunak called Aukus “the most significant multilateral defense partnership in generations.” He said that Britain also will share its 60 years of experience running its own submarine fleet with Australian engineers “so they can build their own fleet.”

The San Diego Union-Tribune reported that Washington. hasn’t made this kind of deal since it helped Britain with nuclear submarine development in 1958 during the Cold War.

In January, Australia announced plans to purchase advanced sea mines to safeguard its maritime routes as the Chinese threat in the Pacific grows.

In a joint statement before the formal announcement, the three leaders said their countries have worked for decades to sustain peace, stability, and prosperity around the world, including in the Indo-Pacific.

Australia is buying three, and possibly up to five, Virginia-class fast-attack submarines as part of Aukus. A future generation of submarines will be built in Britain and in Australia with American technology and support. America will also increase its port visits in Australia to provide it with more familiarity with the nuclear-powered technology before it has such subs of its own. The USS Asheville was docked at Perth, Australia, on Monday, Mr. Biden said.

Mr. Sunak invited Mr. Biden, who is of Irish descent, to visit Northern Ireland in April to help celebrate the 25th anniversary of its peace accord, the 1998 Good Friday agreement. Mr. Biden replied that it is his “intention” to go to both Northern Ireland and the Republic of Ireland. During a subsequent meeting with Mr. Albanese, Mr. Biden said he planned to visit Australia in May for meetings that will include the leaders of Japan and India.

Mr. Biden views the partnerships and alliances in the region as cornerstones for American strategy for years to come. Asked if Aukus would survive if a new, more isolationist president was elected — a veiled reference to Donald Trump, who is running for another term — Mr. Biden said yes.

The secretly brokered Aukus deal included the Australian government’s cancellation of a $66 billion contract for a French-built fleet of conventional submarines, which sparked a diplomatic row within the Western alliance that took months to mend.

China has argued that the Aukus deal violates the Nuclear Non-Proliferation Treaty. It contends that the transfer of nuclear weapons materials from a nuclear-weapon state to a non-nuclear-weapon state is a “blatant” violation of the spirit of the pact. Australian officials have pushed back against the criticism, arguing that they are working to acquire nuclear-powered, not nuclear-armed, submarines.

“The question is really how does China choose to respond because Australia is not backing away from what it sees to be doing in its own interests here,” a senior adviser and Australia chairman at the Center for Strategic and International Studies, Charles Edel, said. “I think that probably from Beijing’s perspective they’ve already counted out Australia as a wooable mid country. It seemed to have fully gone into the American camp.”

San Diego was Mr. Biden’s first stop on a three-day trip to California and Nevada. He will discuss gun violence prevention at Monterey Park, California, where 11 people were killed in a January mass shooting, and his plans to lower prescription drug costs at Las Vegas. The trip will include fundraising stops as Mr. Biden steps up his political activity before an expected re-election announcement next month.

A fundraiser in Rancho Santa Fe, California, on Monday night was expected to include about 40 attendees and raise $1 million for Democrats, according to a Democratic National Committee official.