Mr Cook said he had been talking to Hashim Thaci, the head of the political directorate of the Kosovo Liberation Army.

"He believes that there are some 400,000 living in the mountains and woods hiding from the Serbs who would otherwise kill or deport them," said Mr Cook.

Mr Thaci told Mr Cook that the refugees were in a very serious condition, and were particularly short of food.

Before the air strikes began, there were an estimated 250,000 internally-displaced people within Kosovo.

Mr Cook said the responsibility for the refugees' welfare lay squarely on Yugoslav President Slobodan Milosevic.

New wave of expulsions

Meanwhile, more than 4,000 Kosovo Albanian refugees streamed into northern Albania on Saturday night after Yugoslavia re-opened its main border crossing.

Kevin Connolly reports from the border: "New evidence of renewed terror"

International monitors say they were forced out of Kosovo in what some refugees are calling another wave of ethnic cleansing.

Hundreds of thousands of Kosovo Albanians have been driven out of the province since Serbian forces intensified a campaign of ethnic dispossession in response to the Nato bombing campaign launched on 24 March.

(Click here for map of latest refugee movements)

And the UK's Department for International Development says that as many as 100,000 missing Kosovo Albanian men may have been killed.

DFID staff who have just returned from Macedonia said that some refugee camps were almost devoid of men.

Border re-opened

The influx by bus, car and tractor is the second major flow of refugees since Wednesday, when Yugoslavia abruptly closed the frontier.

Humphrey Hawkesley: "Nato has failed to save Kosovo, Milosevic has failed to create a Greater Serbia"

According to the monitors, people from the village of Vragolja near the Kosovo capital Pristina, say Serb troops arrived on Saturday morning.

They were reportedly told that if they wanted to become Serb they could stay, if they wanted to remain Albanian they had to leave.

Hundreds of refugees have also been arriving in Macedonia, although at one crossing point, the Yugoslav authorities turned back a line of vehicles that eyewitnesses said stretched for miles.

Reporting from the Albanian-Kosovo border, BBC correspondent Duncan Kennedy says it is not clear whether the two latest waves of refugees means a resumption of full scale expulsions or are isolated examples.

According to the UNHCR, plans to airlift refugees to temporary camps in countries in North America and Australia have been put on hold.

UN spokeswoman Paula Ghedini said evacuations to European countries would continue.

KLA clashes

Further north, international monitors have reported gunfire between Yugoslav forces and guerrilla fighters from the Kosovo Liberation Army.

Observers described the situation as anything but calm.

It was also reported that some mortar shells had landed inside Albania, an event that the Albanian authorities have described as provocative.