There is growing international concern about the situation within Kosovo with hundreds of thousands of Kosovo Albanians said to be trapped by Serb forces in enclaves within Kosovo.

The UN agency, the World Food Programme, estimates 800,000 people have left their homes but remain in the province.

According to Nato officials, hundreds of thousands of Kosovo Albanians are trapped in five areas of the province.

The Kosovo Liberation Army says Serb troops are still clearing villages at gunpoint. They say 35,000 refugees are trapped in the mountains north of Pristina, facing both starvation and artillery bombardment from the Serbs.

'Surge' strategy

Nato says that during recent days the Yugoslav forces in Kosovo have been reinforced and that they seem to be redoubling their campaign of ethnic cleansing.

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Spokesman Jamie Shea said the Serbs were mounting a ''safari operation'' against Kosovo-Albanians, forcing them down from the hills and into convoys and trains.

He said Serb strategy appeared to be to drive people to the south of Kosovo, herding them near to the border, but not allowing them to cross. ''It is as if Milosevic is trying to develop a surge operation,'' he said.

After a lull in the flow of refugees out of Kosovo, the exodus resumed on Monday, causing havoc in the already-overcrowded Macedonia.

As many as 7,000 ethnic Albanian refugees were stranded in a no-man's land between the two countries, cold, hungry and traumatised, as the Macedonian authorities said their camps were already full.

Jonathan Charles: "The Macedonian auythorities don't want anyone to witness the condition of refugees"

But after talks between the UN refugee agency and Macedonian officials, the authorities have now agreed to let refugees in.

A BBC correspondent at the scene says the UN is now scrambling to construct a new camp for the growing number of refugees.

The UNHCR said it expected another 100,000 refugees to flood out of Kosovo in the next few days.

'Boy prisoners'

At the daily Nato briefing, Mr Shea said refugees reported that Serb forces were holding 700 ethnic Albanian boys prisoner, using them as "blood banks" for their injured troops.

The BBC's Brian Barron: "A convincing picture of ethnic cleansing"

He said upwards of 100,000 Albanian men of fighting age were unaccounted for in Kosovo. Reports from refugees suggested more than 3,500 had been executed, he said.

BBC Defence Correspondent Jonathan Marcus says Belgrade's policy of ethnic cleansing has become its most potent weapon:

• threatening to destabilise neighbouring countries

• forcing Nato to mount a huge relief operation that inevitably slows the movement of military forces into the region

• flooding the roads of south western Kosovo with refugees, greatly complicating the task of Nato pilots

Serbian reports speak of 20,000 refugees returning voluntarily to their villages near Podujevo in northern Kosovo, after having been forced out by Nato bombings and separatist attacks.

Last push

Because of the increasingly desperate plight of the refugees within Kosovo, France said on Monday that it was preparing equipment that would be needed for an airdrop of supplies to desperate refugees trapped inside Kosovo.

However, Nato has said any planes dropping supplies could be "sitting ducks" for the military.