The BBC's James Robbins reports: "The technology is still top secret"

The Scotsman - published in Edinburgh - alleges that an unidentified Nato officer leaked top secret flight plans and mission targets to Russian military intelligence.

Moscow then passed the information on to the Serbs, according to the article, which was written by military analyst Paul Beaver.

"The Serbs are claiming that they managed to shoot down the fighter because they had information about its track - and they were able to set what's known as a SAMbush, a surface-to-air ambush", Mr Beaver said on Friday.

No comment

The Russians have declined to comment.

'Low Observable Aircraft'

"Like any other intelligence service in the world, our service does not comment as to whether this or that individual belongs or does not belong to its personnel or agent structures", a spokesman for Russian Military Intelligence (SVR) told the Interfax news agency.

The shooting down of the Nighthawk aircraft on 27 March was highly embarrassing for Nato and the US defence establishment.

The wreckage of the fighter was shown repeatedly on Serbian state-run television.

Paul Beaver: "With information about the plane's track, the Serbs were able to set an ambush"

The pilot ejected safely and was rescued by Nato search-and-rescue teams - narrowly eluding attempts by the Yugoslav army to capture him.

The Pentagon has been quoted as rejecting the Scotsman claim, saying that the "tasking orders" for American missions did not go through Nato channels.

The article says the alleged spy was an officer high up in Nato's Brussels command structure, with access to highly-sensitive documents.

"Russian military intelligence was passing on information to the Serbs during the war," said Russian defence analyst Pavel Felganhauer.

"But it would be strange for them to compromise such a well-placed spy just to get one plane."

Mr Felganhauer told the BBC it was possible that a lucky strike by Serb air defences had been turned into an opportunity to "reduce the effectiveness of Nato staff work" by starting a hunt for an imaginary spy.

'Low observable' aircraft

The secret of the fighter's success is "stealth technology" - special composite materials designed to minimise the plane's profile to enemy radar systems.

Pavel Felganhauer: "The Serbs could be trying to reduce Nato effectiveness by starting a spy-hunt"

Aerospace analysts say the planes - known as "Low Observable" aircraft - are not actually invisible to radar or infra-red sensors.

But the special materials - coupled with absorbent paint - make them very hard to locate.

Normally, by the time they are detected - if that happens - the attack has already taken place.

A single-seater, the F-117A was originally conceived in the late 1970s.