US President Donald Trump's administration has launched a formal review of a defence pact that former president Joe Biden made with Australia and the United Kingdom allowing Australia to acquire conventionally armed nuclear submarines, a US defence official told Reuters.

The launch of the formal, Pentagon-led review is likely to alarm Australia, which sees the submarines as critical to its own defence as tensions grow over China's expansive military buildup.

It could also throw a wrench in Britain's defence planning. AUKUS is at the centre of a planned expansion of its submarine fleet.

"We are reviewing AUKUS as part of ensuring that this initiative of the previous administration is aligned with the President's America First agenda," the official said of the review, which was first reported by Financial Times.

"Any changes to the administration's approach for AUKUS will be communicated through official channels, when appropriate."

AUKUS, formed in 2021 to address shared worries about China's growing power, is designed to allow Australia to acquire nuclear-powered attack submarines and other advanced weapons such as hypersonic missiles.

Vocal skeptics of the AUKUS deal among Trump's senior policy officials include Elbridge Colby, the Pentagon's top policy adviser.

In a 2024 talk with Britain's Policy Exchange think-tank, Colby cautioned that US military submarines were a scarce, critical commodity, and that US industry could not produce enough of them to meet American demand.

They would also be central to US military strategy in any confrontation with China centred in the First Island Chain, an area that runs from Japan through Taiwan, the Philippines and on to Borneo, enclosing China's coastal seas.

"My concern is why are we giving away this crown jewel asset when we most need it," Colby said.

The Australian and UK embassies in Washington did not immediately respond to a request for comment.

The US National Security Council also did not immediately respond to a request for comment.

Only six countries operate nuclear submarines: the US, the UK, Russia, China, France and India.

AUKUS would add Australia to that club starting in 2032 with the US sale of Virginia-class submarines. Before that, the US and Britain would start forward rotations of their submarines in 2027 out of an Australian naval base in Western Australia.

Later, Britain and Australia would design and build a new class of submarines, with US assistance, with the first delivery to the UK in the late 2030s and to Australia in the early 2040s.

Although Australia has declined to say ahead of time whether it would send the submarines to join US forces in any conflict between the US and China, Colby noted Australia's historic alliance with Washington, including sending troops to Vietnam.

"I think we can make a decent bet that Australia would be there with us in the event of a conflict," Colby said last year.

Speaking in Congress on Tuesday, Defense Secretary Pete Hegseth said "we're having honest conversations with our allies."

On Australia, Hegseth said: "We want to make sure those capabilities are part of how they use them with their submarines, but also how they integrate with us as allies."