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Macedonia is tightening its border with Kosovo as Nato launches a major operation to save the lives of hundreds of thousands of ethnic Albanian refugees who have fled Kosovo.

An estimated 50,000 refugees are stuck at the Blace border post in freezing temperatures after the former Yugoslav republic said it could not take any more people. Disease is said to be spreading.

About 315,000 Kosovo Albanians have fled or have been expelled since the air strikes began 11 days ago. Nato says at this rate the Serbs will empty their southern province within another 20 days.

The BBC has been shown the first known video evidence of the alleged Serbian repression in Kosovo.

George Alagiah reports on video film from Kosovo. WARNING: PICTURES MAY BE DISTURBING

The pictures - taken by a Kosovo Albanian - show up to 100 bodies of men allegedly massacred by Serb forces in the Kosovo village of Krusa.

UK Foreign Secretary Robin Cook reacted by saying: "This is an appalling event. It underlines the murderous brutality with which the ethnic cleansing is being waged within Kosovo."

Nato has announced it is changing its military strategy, to demand the withdrawal of all Serbian forces from Kosovo.

Ben Brown on the Kosovo-Macedonia border: "Disease is spreading and people are dying"

The alliance says one million meals are needed to feed refugees. It is also considering sending helicopters into Albania to shuttle aid into the north and transfer refugees south.

Mr Cook has proposed creating official sanctuaries within Macedonia to deal with the crisis.

"The proposal has been greeted with interest by the Macedonian Government," he said.

"The people in direst needs are those who have not still not been allowed through the border."

At Blace, there have been reports of fights for what little food is available and security forces have been struggling to maintain control.

Battles for food: Supplies are scarce

Some refugees have threatened to march out of the camp into Macedonia.

Macedonian security forces are standing by in riot gear with automatic weapons to prevent any unauthorised move from the border area.

In a statement issued after the first deaths among refugees - 11 since Friday night including four babies - Macedonian President Kiro Gligorov said it would only allow entry to those who had guaranteed passage to other countries.

Earlier, Nato escalated its bombing campaign in Yugoslavia, hitting the centre of Belgrade for the first time.

Two ministries described by Nato as the "brain" of the Serb ethnic cleansing operation in Kosovo were destroyed by Cruise missiles.

Italy has announced that a meeting of the six-nation Contact Group on Yugoslavia would discuss the crisis next week, at Russia's request.

Agencies under strain

Hannu Pekka Laiho of the Red Cross: "People are really suffering"

As aid agencies struggled to cope with the deepening refugee crisis, Nato announced that it was setting up a headquarters in Albania to co-ordinate the humanitarian effort. It will be staffed by 6,000 Italian troops.

Latest estimates from the United Nations Commissioner for Refugees report that 170,000 have fled or have been expelled to Albania and 115,000 to Macedonia since 24 March.

Security cordon: Armed police have surrounded camp

Aid agencies have accused the Macedonian Government of deliberately hampering the flow of refugees with appalling consequence for those stuck on the border.

One British charity, Children's Aid, is leaving Macedonia and taking its food supplies to Albania in protest at the obstructions.

UK Prime Minister Tony Blair warned Yugoslav President Slobodan Milosevic that Nato would reverse the expulsion of Kosovo's Albanians.

In a pledge to the refugees, he said: "We will not let you down. We will make sure that you are able to return to your homes, and live in your homes in peace.

"That is our promise to you and we will carry it out."

President Clinton: "One in three have been made homeless"

Speaking in his weekly radio address to the American people, President Bill Clinton said: "Our nation cannot do everything. We can't end all suffering, we can't stop all violence.

"But there are times when looking away simply is not an option.

"Our goal is to exact a very high price for Mr Milosevic's policy of repression."