The US has selected a design for the futuristic "Golden Dome" missile defence system, says US President Donald Trump, adding that it will be operational by the end of his time in office.

Just days after returning to the White House in January, Trump unveiled his intentions for the system, aimed at countering "next-generation" aerial threats to the US, including ballistic and cruise missiles.

An initial sum of $25bn (£18.7bn) has been earmarked in a new budget bill - although the government has estimated it could end up costing 20 times that over decades.

There are also doubts about whether the US will be able to deliver a comprehensive defence system for such a huge land mass.

Officials warn that existing systems have not kept pace with increasingly sophisticated weapons possessed by potential adversaries.

A briefing document recently released by the Defense Intelligence Agency noted that missile threats "will expand in scale and sophistication", with China and Russia actively designing systems "to exploit gaps" in US defences.

Seven days into his second administration, Trump ordered the defence department to submit plans for a system that would deter and defend against aerial attacks, which the White House said remain "the most catastrophic threat" facing the US.

Speaking in the Oval Office on Tuesday, Trump said the system would consist of "next-generation" technologies across land, sea and space, including space-based sensors and interceptors.

Trump added that the system would be "capable even of intercepting missiles launched from the other side of the world, or launched from space".

The system is partly inspired by Israel's Iron Dome, which the country has used to intercept rockets and missiles since 2011.

The Golden Dome, however, would be many times larger and designed to combat a wider range of threats, including hypersonic weapons able to move faster than the speed of sound and fractional orbital bombardment systems - also called Fobs - that could deliver warheads from space.

"Israel's missile defence challenge is a lot easier than one in the United States," Marion Messmer, a senior research fellow at London-based Chatham House, told the New York Times. "The geography is much smaller and the angles and directions and the types of missiles are more limited."

Shashank Joshi, defence editor at the Economist, told the BBC the Golden Dome would probably work by using satellites to track missiles and then intercepting them as they took off.

He said the US military would take the plan very seriously but it was unrealistic to think it would be completed during Trump's term, and the huge cost would suck up a large chunk of the US defence budget.

Trump said on Tuesday that the programme would require an initial investment of $25bn, with a total cost of $175bn over time.

The Congressional Budget Office, however, has estimated that the government could ultimately spend more, up to $542bn over 20 years, on the space-based parts of the system alone.

Trump said Canada had asked to be a part of the system.

During a visit to Washington earlier this year, then-Canadian defence minister Bill Blair acknowledged that Canada was interested in participating in the dome project, arguing that it "makes sense" and was in the country's "national interest".

The many aspects of the system will fall under one centralised command, US defence officials have said. Space Force General Michael Guetlein will oversee the project.