There have been conflicting claims about who was responsible, with Belgrade claiming Nato fired on a column of cars and tractors.

Nato said air crews involved believed they had attacked a military convoy, which had fired on them.

Spokesman Jamie Shea said the crews broke off attacks when they realised there were civilian vehicles on the same road.

Sadaf Maruf: "Nato has stressed it was aiming at military targets"

"Nato pilots attack only military vehicles. I don't know yet to what degree we may have caused collateral damage," he said.

General Wesley Clark, who commands the Nato operation, said there were reports that Serb forces had attacked the civilians, but the Pentagon then backed away from these claims.

Marko Gasic, spokesman for the Serb Information Exchange, told the BBC that the convoy had been returning to the town of Djakovica when it was attacked twice by Nato bombers.

Our correspondent says the confusion is heightened because there are reports of a second attack, involving Serb aircraft.

"There are also timing discrepancies, such as the Nato air strike being two hours after one of the convoys was hit," he said.

The BBC's Jeremy Bowen: "All the refugees I spoke to said there were no Serb military vehicles"

Nato officials are analysing cockpit video footage from the aircraft involved in the raid, as well as interviewing the pilots.

The Serbian authorities are also taking a group of western journalists to the site of the attack, near the town of Djakovica.

A Danish military officer, part of a team monitoring the Albanian border, said refugee accounts indicated that Serbs, using MiGs and helicopters, had taken advantage of a Nato military strike to deliberately target refugees.

Bombing intensifies

During a night of clear skies, Yugoslavia was subjected to one of the heaviest air strikes since Nato raids began three weeks ago.

(Click here for a map showing Nato's latest strikes).

Up to six powerful explosions shook the capital, Belgrade, shattering windows and tearing doors from their frames.

Defence Correspondent Mark Laity: "It looks like there were two separate convoys"

Serbian media said army barracks at Racovica about 8km from the city centre had been hit, and power supplies to the region cut.

Nato air strikes destroyed two bridges in central and southern Serbia, one of them leading to the Kosovo capital of Pristina.

Pristina was also reported to be under heavy bombardment. Two transmitters were hit, cutting programmes of the state-run television.

As the bombing intensified, German proposals for a possible 24-hour pause in the air campaign to allow Serbian forces to begin a withdrawal were greeted cautiously by Nato.

• Cook damns Belgrade's 'crocodile tears'
• Annan searches for Kosovo peace
• Milosevic breaks his silence
• KLA strongholds under attack
• B92 under new management