It would allow the US to carry out strikes on possible enemy missile sites at short notice without relying on overseas bases.

As part of a massive $459 billion spending bill for next year, $100 million has been set aside to develop what is described as the "prompt global strike program".

The new program envisages sending a vehicle into space that could launch a precision-guided warhead anywhere in the world within two hours.

The vehicle, which would fly at hypersonic speeds - which is faster than supersonic - would be reusable and able to return back to Earth.