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, although Nato said it could not confirm they were being fired on.

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.

With the flow of refugees having slowed to a trickle on Monday, fears increased that the Serb forces were driving people back from the borders.

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.

Spokesman Jamie Shea said 70,000 refugees had left Kosovo over the weekend, and a total of 600,000 had departed in the past month.

'Safari operation'

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.

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.

Mr 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.

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.

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

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

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.

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.

Macedonian border

Rageh Omaar in Skopje, Macedonia: The camps are at full capacity

In Macedonia, the UN refugee agency UNHCR declared camps in Macedonia full beyond capacity. The Macedonian Government says it will only admit 400 of the 3,000 refugees that arrived on Monday night.

Reports say the refugees that arrived at the Macedonian border overnight are camped immediately on the border - cold, terrified and with no sanctuary.

The UN has been unable to get aid to them.

The UN refugee agency has asked the Macedonian government to be allowed to build three new refugee camps. The authorities have said they will allow one small additional camp to be built and no more.

The World Food Programme says about 100,000 of the internally-displaced people inside Kosovo are heading for Macedonia.

They say another 175,000 are travelling towards Montenegro.

A spokeswoman in Albania said it was unclear where the rest were going, but they were probably trying to flee Serb tanks.

"They are heading towards the mountains, I think, to escape the tanks. I'm sure that some of those people will eventually wind up here in Albania," said Angela Walker.

A spokeswoman for the UN children's agency says fewer than half of the Kosovo Albanians have been vaccinated, due to a widespread mistrust of the Yugoslav-run health authorities.