The UN chief also called on Yugoslavia's neighbours to keep their borders open to Kosovo Albanians as they fled Serb forces.

At one point on Tuesday the former Yugoslav republic of Macedonia had in effect closed its frontier with Kosovo as thousands of ethnic Albanians lined up to cross the border.

"Once again, the civilian population is being made to pay the price for an unresolved political dispute," Mr Annan said.

Robert Parsons : Thousands are lining the Macadonian border, waiting for permission to move on

They "must never come under indiscriminate and deliberate attack. Such actions are in flagrant violation of established humanitarian law."

He added: "I am profoundly outraged by reports of a vicious and systematic campaign of ethnic cleansing conducted by Serbian military and paramilitary forces in the province of Kosovo."

Border crossings

In Macedonia, official estimates suggested over 25,000 people were seeking refuge and the government appealed for more aid to help the refugees.

The Macedonian army and security forces on Tuesday allowed thousands of refugees off a mountainside where they had been stranded.

More than 3,000 people had been trapped there by the Yugoslav army on one side and Macedonian forces on the other.

Some refugees reported two infants, a young man and two elderly people died there.

BBC's Paul Welsh speaks to women and children fleeing to Montenegro

Those seeking to flee Yugoslavia by car were making slow progress at the border.

By late Tuesday evening a line of cars three miles long was reported to have formed at the Blace border crossing, just north of the Macedonian capital.

But only one car was allowed to pass every 25 minutes.

One driver said thousands had left Pristina by car on Sunday and Monday, taking what they could in their cars.

Serb police 'helping'

Refugees are flooding into Albania

Although they had experienced some trouble at Yugoslav police checkpoints on the road, they were allowed to pass.

"It's almost as if they organised this. Police are helping. Many times on the road they say faster, faster," said Valdan Haliti.

Macedonian officials said they were taking measures to cope with the influx.

Plans were reported to be under way to set up three refugee centres near the main border crossings, and meetings had been held with envoys from neighbouring countries to discuss ways to move on some of the refugees.

The BBC's Paul Wood in Skopje: Refugees are crossing mined mountain tracks

More than 120,000 Kosovo Albanians have now fled Kosovo amid continuing reports of Serb forces assaulting civilians and burning houses.

Reports say another 100,000 are still surging towards the borders of Montenegro, Macedonia and Albania.

Paul Stromberg, a Geneva-based spokesman for the United Nations refugee agency, UNHCR, said reports from observers on the borders spoke of widespread violence and intimidation from Serb forces.

"The picture drawn for us by refugees is very, very grim and very, very discouraging," he said.

Albania struggles to cope

The Albanian President, Rexhep Meidani, has also urged the international community to help his country cope with a massive influx of refugees.

Refugees reach for water from aid workers

The border town of Kukes saw its 18,000 inhabitants swamped by more than 80,000 refugees in 72 hours.

In Montenegro, correspondents have described the stream of refugees as an exodus and officials there expect 40,000 more to arrive.