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

"In the campaign I promised the American people I would build a cutting-edge missile defense 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 $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 $161 billion and $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."

Trump said the defense shield should be operational by the end of his term in January 2029, but industry experts were less certain of that timeframe and the cost. 

"Ronald Reagan wanted it many years ago, but they didn't have the technology," Trump said, referring to the space-based missile defense system, popularly called "Star Wars", that Reagan proposed.

The Golden Dome program faces both political scrutiny and funding uncertainty.

"The new datapoint is the $175 billion, but the question remains, over what period of time. It's probably 10 years," said Tom Karako of the Center for Strategic and International Studies.

Silicon Valley and US software expertise can be leveraged to bring advances, while also using existing missile defense systems, he added.

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."

Russia, China oppose Golden Dome 

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

The United States faces various missile threats from adversaries, but they differ significantly from the short-range weapons that Israel's Iron Dome is designed to counter.

The 2022 Missile Defense Review pointed to growing threats from Russia and China.

Beijing is closing the gap with Washington when it comes to ballistic and hypersonic missile technology, while Moscow is modernizing its intercontinental-range missile systems and developing advanced precision strike missiles, the document said.

It also said that the threat of drones -- which have played a key role in the Ukraine war -- is likely to grow, and warned of the danger of ballistic missiles from North Korea and Iran, as well as rocket and missile threats from non-state actors.

Russia and China earlier this month slammed the Golden Dome concept as "deeply destabilizing," saying it risked turning space into a "battlefield."

It "explicitly provides for a significant strengthening of the arsenal for conducting combat operations in space," said a statement published by the Kremlin after talks between the two sides.

The United States has gained valuable real-world experience in defending against missiles and drones in recent years.

In Ukraine, US systems have been used to counter advanced Russian missiles, while American planes and warships helped defend Israel against Iranian attacks last year and have repeatedly shot down missiles and drones launched at ships by Yemen's Tehran-backed Huthi rebels.