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

An AFP journalist, who saw the wreckage of the bus an hour after it was struck by Nato, says he counted 24 bodies. Serb media say the death toll could be around 40.

Nick Childs at Nato says the issue of civilian casualties remains sensitive

The bus was split in two - half of it remained burning on the bridge an hour later and half plunged into the valley.

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

Another bomb appears to have 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.

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.