Defence Minister Richard Marles has held high-level talks with his US counterpart as he mulls over which nuclear-powered submarines to buy.

Mr Marles is making the final decisions on whether Australia will sign up for a US or UK nuclear submarine design under the trilateral AUKUS alliance, which is set to underpin the nation’s security for decades.

He toured a General Dynamics Electric Boat shipyard where nuclear submarines are built, and also visited the 9/11 memorial on Monday.

Meeting with US Secretary of Defence Lloyd Austin, Mr Marles said a complex and precarious world meant Australia had to look to its allies.

“We see it in Ukraine, with the rules-based order being tested there, but we see it in the Indo-Pacific as well,” he said ahead of the meeting.

“And every step of the way forward is not necessarily obvious, but one of the things that’s really clear is it’s a time to be close with friends.”

Mr Lloyd said discussions would centre around “ambitious steps to further strengthen our unbreakable alliance” in the wake of a more aggressive China and Russia.

“It’s safe to say that the alliance between the United States and Australia is as strong as it’s ever been, and it remains vital to regional security,” he said.

Australia is set to unveil its plans to plug capability gaps in March, outlining the schedule for nuclear submarines to be delivered and whether any off-the-shelf vessels will be provided by the US in the interim.

There has been speculation that Washington is prepared to sell Australia its own submarines to plug any gaps in the meantime in light of China’s increased aggression and the fact that the first nuclear sub is not set to arrive for at least 15 years.

One US lawmaker has cast doubt on the ability to interrupt his nation’s own build schedule to sell Australia the submarine.

“There’s been a lot of talk about, well, the Australians would just buy a US submarine. That’s not going to happen,” Republican Rob Wittman told a local defence media outlet.

“I just don’t see how we’re going to build a submarine and sell it to Australia during that time.”

Mr Marles is in the US with Foreign Minister Penny Wong for the annual AUSMIN talks with their American counterparts.

The US ambassador to Australia, Caroline Kennedy, is also travelling with the pair.

– AAP