The leaders of the United States, Australia and Britain on Monday unveiled details of a plan to provide Australia with nuclear-powered attack submarines, a major step involving investment of hundreds of billions of dollars aimed at countering China's ambitions in the Indo-Pacific.

Addressing a ceremony at the US naval base in San Diego, accompanied by Australian Prime Minister Anthony Albanese and British Prime Minister Rishi Sunak, US President Joe Biden called the agreement under the 2021 AUKUS partnership part of a shared commitment to a free-and-open Indo-Pacific region with two of America's "most stalwart and capable allies."

Sunak called it "a powerful partnership," adding: "For the first time ever it will mean three fleets of submarines working together across the Atlantic and Pacific keeping our oceans free ... for decades to come."

Under the deal, the United States intends to sell Australia three US Virginia class nuclear-powered submarines, which are built by General Dynamics, in the early 2030s, with an option to buy two more if needed, the joint statement said.

The statement from the leaders said the multi-stage project would culminate with British and Australian production and operation of a new class of submarine - SSN-AUKUS - a "trilaterally developed" vessel based on Britain's next-generation design that would be built in Britain and Australia and include "cutting edge" US technologies.

An Australian defence official said the project would cost US$245 billion by 2055.

Biden stressed that the submarines would be nuclear-powered, not nuclear armed: "These boats will not have nuclear weapons of any kind on them," he said.

Britain will take delivery of its first SSN-AUKUS submarine in the late 2030s, and Australia would receive its first in the early 2040s, Albanese and the British statement said.

The agreement will also see US and British submarines deployed in Western Australia to help train Australian crews and bolster deterrence.

The United States and Britain would begin these rotational deployments as soon as 2027, the joint statement said. The US official said this would increase to four US submarines and one British in a few years.

This first phase of the plan is already underway with the Virginia-class nuclear-powered attack submarine Asheville visiting Perth in Western Australia, officials said.

China has condemned AUKUS as an illegal act of nuclear proliferation. In launching the partnership Australia also upset France by abruptly cancelling a deal to buy French conventional submarines.

A professor in politics and international relations in Shanghai told RTHK the announcement shows the US wants to keep up "its Cold War mentality against China".

Josef Gregory Mahoney, from East China Normal University in Shanghai, said Beijing's appointment of Qin Gang as foreign minister and US-sanctioned general, Li Shangfu, as defence minister show that Beijing is holding its ground.

"This is another indication that China is going to take its own path; it's going to promote its own people; and the United States will now be in the difficult position of either dealing with defence minister Li or continuing to blacklist him," Mahoney said.

"Both sides are digging in," he added.

Big questions remain about the submarine plan, not least over strict US curbs on the extensive technology sharing needed for the project and about how long it will take to deliver the submarines. (Additional reporting by Reuters)