The prospect of a swift trial, as promised by some sources in Belgrade, receded when a Serb official in Kosovo said: "The process of collecting evidence, on the basis of which a criminal procedure starts, is under way. More details will be available on Saturday".

It is unclear whether the three have appeared in court, or even what they will be accused of.

Belgrade officials say they could face charges of espionage or of invading Yugoslav territory.

Bill Clinton addresses US servicemen's families and President Milosevic

US President Bill Clinton said there was no basis for them to be put on trial. He delivered a blunt warning to Yugoslav President Slobodan Milosevic that he would be held responsible for their safety and well-being.

The three US soldiers - who were paraded on Serbian television showing some signs of physical injury - were from a peacekeeping unit based in the former Yugoslav republic of Macedonia.

The US said that it regarded the three soldiers as prisoners of war and that it would be outraged if they were put on trial.

Pentagon spokesman Ken Bacon said the three men had been inside the Macedonian border when they radioed they were under fire and surrounded. However, Serbian television reported that the men had been captured on Yugoslav territory.

Stephen Gibbs reports: The men's capture is a propaganda coup for the Yugoslav authorities

Mr Bacon said: "We consider them to be PoWs and ... we believe that, we assert, that they are covered by the Geneva Convention which of course gives them the series of internationally recognised protections."

Yugoslav Deputy Prime Minister Vuk Draskovic said that the soldiers would be treated "with the full respect of all international conventions concerning prisoners of war".

'They don't deserve this'

The three captured men were named as Steven M Gonzales, Andrew A Ramirez and Christopher J Stone.

Jim Stone: "My family and I are very concerned about Chris"

Steven Gonzales' mother, Rosie, said: "Steven and those other two soldiers don't deserve this.

"They're young men with their whole lives ahead of them who were put in a bad situation. We're hopeful for the safe return of all three of them."

In spite of this latest twist in the Kosovo crisis, President Clinton vowed to stay the course. "We will continue to carry out our mission with determination and resolve," he said.