Defence analysts are convinced they are caused by the US Air Force's top-secret Project Aurora-a spyplane as big as Concorde that can fly three times as fast.

The US Air Force is using the remote RAF airbase at Machrihanish, Strathclyde, as a staging point, it was claimed last week The mystery aircraft lies been stealthily dropping in at night before streaking back to America across the North Pole at more than six times the speed of sound.

Flying on the edge of space, powered by a "pulser" engine, the aircraft is being tested in missions--that circle the globe.

Guy Norris of Flight Magazine, who is based In Los Angeles, said last night that Aurora was being flown from a base in the Nevada desert to an atoll in the Pacific, then on to Scotland to refuel before returning to the US at night.

Specially modified tanker aircraft flying from Britain are being used to top up Aurora's tanks with liquid methane fuel in mid-air.

The aircraft carries high-powered cameras and infrared radar which can see down through cloud cover. Its flightpath covers targets in Iraq. An F-111 fighterbomber is scrambled as the black-painted aircraft lands, flying in close formation to confuse prying civilian radars, according to reports from US airmen.

Paul Wiggins, Editor of Scottish Airnews, describes Machrihanish - as 'the Remotest military airfield in Britain' -- with a very long runway." It is guarded by a detachment of US "Seals," the equivalent of Britain's Special Boat Squadron.

"It would be ideal for operating a secret high-perform mance aircraft," he says.

People living nearby reported strange 'ear-splitting' noises and mysterious smoke rings in the sky earlier this year. As rumours grew of secret developments at the base, Archie Hamilton, the Defence Minister, told Parliament last month that the existence of, and any operations by Aurora were a "matter for the American authorities". A US Air Force spokesman yesterday refused to confirm or deny its existence.

But last week independent evidence came from Washington that the Pentagon is spending billions from its "black budget" on a clandestine successor to the SR-71 Blackbird spyplane, retired two years ago.

Kemper Securities, the a financial analysts, said up to $8 billion has already been spent on the project at Lockheed's "Skunk Works" in California where the F-117A stealth fighter was developed.

"Something has been booming Los Angeles regularly for the last six months," said Bill Sweetman, a US based aerospace writer. "The frequency of the sonic booms indicates that whatever is making them is now an operational aircraft."

THE ARTIST'S IMPRESSION IS A CLEAR-CUT BURNELLI CONFIGURATION ALTHOUGH IT IS REFERRED TO AS A "BLENDED WING BODY". THE PENTAGON USES THIS TERMINOLOGY, BECAUSE THE "LIFTING BODY" TERMINOLOGY IS PROPRIETARY TO BURNELLI.