But the alliance said the bridge on which it was travelling was a legitimate military target, and indicated no change in tactics.

The Yugoslav media says more than 40 people died when the bomb hit the bus at Luzane, splitting it in two and scattering bodies over a wide area.

An AFP journalist, who went to the scene an hour after the Nato strike, says he counted 24 bodies.

In a separate attack, five people - including two children - died as bombs hit two villages in Montenegro.

Klaus Naumann: "Bridge repeatedly used by Serbian reinforcements"

"We regret every loss of life, but this happens in military operations and so far we .. (have done) a good job in avoiding civilian casualties," said General Naumann, who is chairman of Nato's military committee.

A Nato communique said the bridge at Luzane, 20km (12 miles) north of Pristina was a key target lying on a north-south supply route used by Serb forces.

Dead bodies were scattered around wrecked bus

"Unfortunately, after weapons release a bus crossed the bridge," the communique said.

A survivor of the attack interviewed in hospital said he had "heard the planes, the blast and then everything started to burn."

BBC Correspondent Jacky Rowland said the bus seemed to have been on a regular passenger service from Nis to Pristina.

The survivor said the bus was filled with civilians, mostly children and old people, and there were a couple of soldiers on board.

Aleksander Mitic of AFP: We counted 24 bodies

According to Yugoslav media, another bomb struck an ambulance which was on the way to the bridge strike, injuring a medical worker.

There is no mention of a second attack in the Nato statement.

A Nato official is quoted as saying the bridge was a secondary target, after the pilot had been unable to bomb his primary target.

It is the second major civilian tragedy caused by a Nato bridge attack, after a train was hit on 10 April with the loss of at least 10 lives.

Five die in Montenegro

In Montenegro, an attack on two villages close to the Kosovo border left five people dead - including two children.

Montenegro's Deputy Prime Minister, Dragisa Burzan, said the government strongly protested against the action, and said that Nato needed to make a distinction between Serbia and Montenegro.

Montenegro is Serbia's smaller partner in the Yugoslav federation, and its government has distanced itself from the federal government of President Slobodan Milosevic.

Saturday's death toll in Kosovo and Montenegro brings to about 200, according to Serb sources, the number of civilians who have died in Nato attacks since the bombardment began in March.