The US Air Force on Wednesday test launched a hypersonic cruise missile, with the vehicle accelerating to Mach 6 before splashing down in the Pacific Ocean, officials said.

The Air Force said the test flight of the X-15A Waverider lasted more than 200 seconds, the longest ever hypersonic flight powered by scramjet propulsion. The previous record was 12 seconds in a NASA X-43 vehicle.

"We are ecstatic to have accomplished most of our test points on the X-51A's very first hypersonic mission," Charlie Brink, program manager with the Air Force Research Laboratory at Wright-Patterson Air Force Base in Ohio.

"We equate this leap in engine technology as equivalent to the post-World War Two jump from propeller-driven aircraft to jet engines," he said.

But about 200 seconds into the flight, "a vehicle anomaly occurred and the flight was terminated," the Air Force said in a statement.

"Engineers are busily examining the data to identify the cause of the problem," it said.

The Waverider was launched from Edwards Air Force Base in California, then carried under the wing of a B-52 aircraft before being released at an altitude of 50,000 feet off the Pacific coast.

A solid rocket booster then propelled the vehicle to about a speed of about Mach 4.8, before the X-51's special scramjet engine ignited.

The Waverider, built by Boeing and Pratt and Whitney Rocketdyne, reached an altitude of 70,000 feet and a top speed of Mach 6, the Air Force said.

Hypersonic flight begins at Mach 5, or five times the speed of sound.

The X-51 fits in with US plans to hit distant targets with conventional weapons within an hour, dubbed "prompt global strike."

The Waverider, or an experimental hypersonic plane also under development, could substitute for a ballistic missile armed with a conventional warhead, as other countries might suspect the missile represented a nuclear attack.