A multi-billion dollar submarine deal between long-standing allies - Australia, the UK and the US - has come under the spotlight after the Trump administration said it was reviewing how the deal fits in with its heavily-touted "America First" agenda.

Congressional Democrats condemned the review as casting doubt over the strength of America's alliances with Britain and Australia.

The United States has announced its AUKUS defence pact with Australia and the United Kingdom has been placed under review. Australian government is projecting confidence that the US will maintain the deal,