The U.S. military lost contact with an unmanned hypersonic test aircraft shortly after its launch, defence officials have revealed.

The Falcon Hypersonic Test Vehicle 2 (HTV-2) - rumoured to be capable of reaching any target in the world in less than an hour - started a test flight from atop a Minotaur IV rocket on Thursday, before officials lost contact with the aircraft.

As a similar incident had already taken place in April, observers now warn the latest problem will come as an embarrassment to the U.S. Defence Advanced Research Projects Agency (DARPA).

The HTV-2, designed to be the fastest ever after aircraft, travelling at 13,000 mph (21,000 kmh), lost contact with ground control soon after being launched from a base in California on Thursday.

According to officials, the plane left Vandenberg Air Force Base in California and managed to separate from its launch vehicle successfully.

However while it was next expected to separate from its rocket near the top of its ascent and then glide back to earth, DARPA said that its monitoring stations lost contact with the aircraft around half an hour into the flight.

The agency had previously said the craft would re-enter the earth's atmosphere over the Pacific Ocean.

It is thus difficult to know whether the goals of the mission were achieved

"Downrange assets did not re-acquire tracking or telemetry. HTV-2 has an autonomous flight termination capacity," the organisation said via social networking site Twitter.

The plane is part of plans from the United States Department of Defence to build what it calls a "prompt global strike" capability, enabling it to hit global targets within an hour with conventional or nuclear warheads.