The alliance said it was unable to shed any more light on confused accounts and contradictory statements concerning the alleged attacks, saying: "When we have information we will share it with you."

The BBC's Defence Correspondent, Jonathan Marcus, says he understands that Nato's military command had prepared a detailed briefing document but at the last moment, Nato's political authorities refused to allow this to be made public.

At a news conference on Friday, Nato spokesman Jamie Shea said: "Nato puts its setbacks behind it ... to use a football term, we're keeping our eye on the ball."

Nicholas Witchell: "Nato in a corner over the refugee convoy"

After repeated questioning he confirmed that a single, laser-targeted bomb dropped by an American F16 hit one civilian target in a convoy travelling north of Djakovica on Wednesday.

He described the destruction of the vehicle, believed to be a tractor, as a "tragic accident".

Serbian officials in Pristina say the incident took place near Meja, adjacent to Djakovica, and claimed more than 60 civilian lives.

Mr Shea said a video of the attack on the refugee convoy would not be made public until Nato had finished its investigation.

But he failed to clear up discrepancies in the number of bombs that were dropped.

The unnamed F-16 pilot responsible for the civilian attack said he dropped one bomb, and then described how a second pilot attacked vehicles in the same area using a laser-guided weapon.

Heated exchanges at the daily Nato press conference

In a second alleged Nato attack, bombs dropped between Djakovica and Prizren to the south are said to have killed six and wounded 11.

Refugee claims that Yugoslav Air Force jets were involved in attacks in the area have not been independently corroborated.

And in a third report, refugees said they saw a convoy bombed between Prizren and the town of Morini.

This attack remains unconfirmed by either Nato or the Yugoslav authorities.

Mr Shea said Nato "does not accept the blame" for either attack.

'Scepticism' about Serb claims

Yugoslav television has been showing pictures of two separate scenes of what they say are attacks on refugee convoys. One shows a column of destroyed vehicles on a tarmac road and the other on a dirt track.

It was not made clear where either site was located.

The alliance has repeatedly cast doubt on the authenticity of many Serb claims.

Journalists taken by Serbian authorities to the scene of one site, between Prizren and Djakovica, were shown evidence of air strikes but say it is impossible to judge the authenticity of Serb claims.

BBC correspondent Michael Williams, who visited the area between Prizren and Djakovica, said that he had seen the bodies of refugees and four impact craters - suggesting four bombs had ben dropped.

This tallies with eyewitness accounts suggesting more than one bomb was dropped on the convoy.

He said there was no evidence of military vehicles, but that 36 hours had elapsed since the bombing raids allowing plenty of time for the scene to be "doctored".