US President Donald Trump unveiled new details on Tuesday on his plan for a "Golden Dome" missile shield system to protect the country against attacks, saying it should be operational in about three years.

The president announced US$25 billion in initial funding for the plan, which he said could eventually cost a total of some US$175 billion.

"In the campaign I promised the American people I would build a cutting-edge missile defence shield," Trump said at the White House. "Today I am pleased to announce we have officially selected architecture for this state-of-the-art system."

"Once fully constructed, the Golden Dome will be capable of intercepting missiles even if they are launched from other sides of the world, and even if they are launched from space," Trump said. "This is very important for the success and even survival of our country."

He said US Space Force General Michael Guetlein will lead the effort, and that Canada has expressed interest in being part of it as "they want to have protection also."

While Trump put the total price at US$175 billion, the Congressional Budget Office has estimated the cost of space-based interceptors to defeat a limited number of intercontinental ballistic missiles at between US$161 billion and US$542 billion over 20 years.

Golden Dome has more expansive goals, with Trump saying it "will deploy next-generation technologies across the land, sea and space, including space-based sensors and interceptors."

Pentagon chief Pete Hegseth, speaking alongside Trump, said the system is aimed at protecting "the homeland from cruise missiles, ballistic missiles, hypersonic missiles, drones, whether they're conventional or nuclear."

The plan's Golden Dome name stems from Israel's Iron Dome air defence system that has intercepted thousands of short-range rockets and other projectiles since it went into operation in 2011. (AFP)