"There has been a worrying change in Serb tactics," she said at a Ministry of Defence briefing.

"They seem now to have started rounding up refugees queuing to leave Kosovo and returning them, by force.

Clare Short: A worrying change in Serb tactics

"We do not know whether they are being driven back to their homes or elsewhere within Kosovo.

"They have also closed the frontier crossing points on the Kosovo side.

"The reason for this change in tactics is not yet clear."

Clare Short: Milosevic will be punished for war crimes

The minister said Britain was trying to gather as much information as possible about the treatment of those barred from leaving Kosovo.

It would try to provide relief to the displaced people, but recognised this would be extremely difficult if they remained in the Serb province.

But Serbian President Slobodan Milosevic had to realise forcing people to move in any direction would not be tolerated, Ms Short said.

She reiterated the United Kingdom's previous pledge to bring President Milosevic to justice for war crimes.

"He and his henchmen will be held fully responsible for any harm that comes to the Kosovars at the hands of his troops and paramilitaries.

"They will be held accountable for war crimes."

The development secretary insisted the only course now open to Nato was to press on with its bombardment, which has entered its third week.

"What we will do is prosecute this war until Serb aggression is reversed, troops withdrawn, an international force put in place and the Kosovars are able to return home safely.

"There will be no compromise, no fudge and no partition of Kosovo.

"No misuse or threat to misuse the people of Kosovo in any way will make us deviate from our objective."

Ms Short said conditions for refugees in Macedonia and Albania were improving, but dangers remained.

"There is no longer a blockage on the Macedonian side of the border.

"Large numbers of people have been moved to the new refugee camp set up by Nato forces at Brazhde, and to other locations in Macedonia and Albania and the provision of relief is improving steadily. This of course is welcome.

"But we remain concerned that refugees in Macedonia are being forced on to planes and buses. This is unacceptable.