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Yugoslavia has closed off its main border with Albania because guards can no longer cope with the swell of refugees escaping the Kosovo crisis.

The action comes as the UK Foreign Secretary, Robin Cook, says Serb forces are carrying out widespread ethnic cleansing in Kosovo, and Russian President Yeltsin sends his prime minister on a peace mission to Belgrade.

Plea for aid

The closure of the Albanian border post came as Albania and Macedonia called for urgent international assistance to deal with refugee flow, the Organisation of Security and Cooperation in Europe (OSCE) announced.

The post at Morina, near Kukes, about 250 km (155 miles) north of the Albanian capital Tirana, was shut after a total of more than 60,000 refugees had arrived.

"There was congestion. There are a lot of people on the other side. Refugees are arriving at a rate of 4,000 per hour now." the spokesman for OSCE's Tirana office said.

While estimates vary as to the precise number of refugees flooding across the borders, authorities agree they stretch into the tens of thousands.

• Albania
  OSCE: 60,000 since Saturday. Now arriving at a rate of 4,000 per hour.
  UNHCR: 30,000 in recent days.
  Government estimate: 60,000 in recent days.
• Macedonia
  UNHCR: 10,000 in recent days.
  Local authorities: 14,000 in recent days.
• Yugoslav Republic of Montenegro
  Government estimate: 2,000 since Saturday.
• Future estimate
  A further 150,000 refugees are thought to be heading for the borders.

A spokesperson for the UN High Commissioner for Refugees, Lyndall Sachs, said giving definite figures was difficult because there were no longer monitors on the ground.

Diplomatic correspondent Bridget Kendall: The fiercest bombing over Kosovo shows Nato's resolve to hit hard

However the UNHCR believes that up to a quarter of the population of Kosovo, more than 500,000 people, have fled their homes since the conflict began.

"This is the figure which has been put together over the past year since this conflict broke out. So a lot of those were displaced before the Nato offensive," Lyndall Sachs said.

A spokesman for the Nato alliance, Jamie Shea, described the situation as the worst Europe had seen for more than 50 years:

"We have to recognise that we are now on the brink of a major humanitarian disaster ... the likes of which we have not seen in Europe since the closing days of World War II," he said.

Serbia denies crisis

But the Serbian Government refugee commissioner, Bratislava Morina, who is of ethnic Albanian origin herself, has dismissed reports of a refugee crisis in Kosovo.

A refugee tragedy unfolding

"There is no humanitarian catastrophe in Kosovo whatsoever," she said in a television interview.

Belgrade says its forces are acting only against the rebel Kosovo Liberation Army.

After five nights of air strikes, there is no sign that Serbian President Slobodan Milosevic is prepared to back down in the face of Nato's air campaign.

The Russian Prime Minister Yevgeny Primakov said 1,000 civilians had so far been killed in the Nato action.

Montenegro welcomes refugees

BBC correspondents in the region say many more thousands of refugees are massing near the borders with stories of their relatives being killed and their houses being looted and burnt.

Paul Welsh: Refugees say they were forced to leave at gunpoint

A steady stream of villagers is now making its way out of Kosovo into neighbouring Montenegro, BBC Correspondent Paul Welsh reports.

The refugees say they were forced to leave at gunpoint. Most of those leaving are women and children, and they say that men were taken away by the armed forces.

Terrified refugees speak of Serb atrocities

Refugees from the town of Pec said security forces gave them five minutes to leave the town at midday on Saturday, or be shot.

Thousands fled the town but some said they had seen bodies in the streets of Pec, and on the roads leading to the border.

Montenegro is Serbia's junior partner in the Yugoslav federation, but is increasingly distancing itself from its neighbour.

Macedonia: Exodus speeds up

The flow of displaced people from Kosovo into Macedonia has increased dramatically in the past 24 hours, according to BBC Correspondent Paul Wood, who has been at the border.

Refugees have spoken consistently of Serbian atrocities. They tell stories of whole families being taken from their homes and shot, adding that men of fighting age are particular targets. Others talk of cases of mutilation.

Paul Wood reports: " A campaign of genocide"

The majority of the refugees are elderly men and women and women with young children. They say they have left behind members of their families and fear for the fate of their menfolk.

Albania: 'Scientific' expulsions

A senior local government official in northern Albania said the Serbs were "scientifically" expelling Kosovo Albanians from their homes, and pushing them towards the border.