Nato has confirmed attacking the bridge 250km (155 miles) south-east of Belgrade, but is still investigating the reports of civilian deaths.

Military authorities in Belgrade say the train was crossing a deep river gorge when Nato planes attacked a nearby road bridge, severing electricity cables and cutting power to the train.

The Serbian military says the train was trapped on the rail bridge and was then hit by a second bombing run.

Two carriages were reported to have burst into flames and another two coaches thrown from the track.

Nato said the railway bridge was a vital supply line for Serbian troops in Kosovo - but there was no intention of hitting the train.

Nato spokesman Jamie Shea: "Nato goes to extraordinary lengths to avoid collateral damage"

Spokesman Jamie Shea said: "Nato aircraft have struck a railway bridge at Grdelica, which is an important part of the supply lines for the Yugoslav military and security forces in Kosovo.

"The reports that we are receiving at the moment indicate that there was a train on or near the bridge at the time of the strike.

"I want to stress very strongly indeed that there was no intention whatever to cause damage to the train. Regrettably we cannot at the moment exclude the possibility of casualties."

Local government officials in Grdelica say nine people died and many more were injured.