The observer, who has a lengthy Armed Forces background but has asked not to be identified, says he first noticed the planes as they passed over the nearby hills.

He then watched from his car as they performed manoeuvres in close formation over Alnwick Moor and the Vale of Whittingham area at dusk last Wednesday.

But he has been left baffled by the sighting, because he couldn't identify the aircraft as anything currently being flown by either British, European or US air forces.

He said: "I could hear the engine noise of jets, which isn't uncommon here in rural Northumberland, but I always look up anyway.

"Instead of the usual RAF Tornados or Hawk trainer jets, I saw two black triangle-shaped objects flying low and fast towards the west.

"They were performing manouevres in tight formation as they went, but their shape didn't conform to any military aircraft I'm familiar with.

"They were very flat in profile and didn't appear to have any tail as such, just what seemed to be small angular fins towards the rear of each 'wing'.

"I've checked with various sources, but can't find anything that even remotely matches what I saw, not even the USAF's F-117 Stealth Fighter.

"It was all very strange."

A spokesman for the Ministry of Defence at the Otterburn Training Area said: "We're not aware of any foreign forces exercising in the area, we just have our normal guys up here at the minute."

Inquiries to RAF Boulmer also drew a blank.

The sighting came just days after RAF top brass admitted that one of their Tornado jets, on a routine training flight from Lossiemouth in Scotland, had lost a dummy bomb somewhere over Northumberland.

And it's almost a year since an Ashington man snapped a photo of a similar-looking object from Tynemouth Pier, which some have speculated was actually a secret US reconnaisance spy plane called the TR-3A Black Manta.

Officially denied by the authorities, the TR-3A is claimed to have a stealthy flying-wing design and is alleged to have been used in the Gulf War to provide laser targeting for F-117A stealth fighters.

Some aviation websites speculate that Black Mantas could now even be flying from remote air stations in the UK.

Critics, however, claim that the TR-3 was nothing more than a prototype for the now fully operational B-2 Spirit stealth bomber.