Using some of his most uncompromising language since the Kosovo crisis began, Mr Annan said: "The brutal persecution the refugees and displaced persons are suffering, the loss of their family members, their homes and even documentation of their identities, underline the urgency of their plight."

Mr Annan told UN Security Council members: "Yet again we face the abominable practice of 'ethnic cleansing' only a few years after it transformed the demography of Bosnia and Herzegovina.

"The Serbian authorities must halt such actions."

The UN High Commissioner for Human Rights, former Irish President Mary Robinson, was "taking urgent steps to strengthen our capacity to monitor the shocking violations of human rights of the Kosovo population by Serb security forces", he said.

But Mr Annan acknowledged that it was "impossible" at present to send observers to the province.

400,000 displaced

The UN estimates that close to 400,000 refugees have left their homes in Kosovo since Nato began its air strikes on Yugoslavia on 24 March.

Those displaced include 226,000 people now in Albania, 120,000 in Macedonia and 35,700 in Montenegro.

Up to 400,000 people are thought to have left Kosovo

The UN secretary-general urged all countries to provide shelter for the refugees until they are able to return to their homes in security.

He said he had been in touch with many world leaders in recent days, including Macedonia's President Kiro Gligorov, who was concerned about the impact of the refugees on the "precarious internal situation in his country".

Mr Annan said he assured the Macedonian president that the international community would share the burden.

Nato on Sunday announced plans by most of its member states to temporarily take in refugees to ease the pressure on the region.

Mr Annan said the offer of the alliance's logistic resources in helping deal with the mass exodus "will make an enormous contribution".

Nato 'aggression'

But the UN secretary-general came under fire from Yugoslav Foreign Minister Zivadin Jovanovic, who said Mr Annan would be party to undermining the UN if he made no attempts to halt Nato's "aggression".

"I have been deeply disappointed that, despite our appeal and the efforts of ... peace-loving member states, the Security Council has failed to act in accordance with its responsibilities as set forth in the (UN) charter."

Nato member states have begun airlifting refugees

Mr Jovanovic called on the UN secretary-general to "urgently act within your powers ... to put an end to the Nato aggression and the further endangerment of international peace and security without delay".

He told Mr Annan in a letter: "You are faced with a historic opportunity to take the side of justice and law, and, at the threshold of a new century, to protect the authority of the United Nations."

The alternative was that Mr Annan would become "an accomplice in undermining the system of the United Nations", Mr Jovanovic warned.

"I hope that it is still not too late for you to make the right choice."

'Deepest concern'

The UN Security Council is divided over Nato strikes

The UN Security Council steered clear on Monday of blaming any party for the refugee exodus, although it did express "deepest concern at the grave humanitarian situation".

The 15-nation Security Council remains divided on the issue of Nato's bombing of Yugoslavia.

Only Russia, China and Namibia voted in favour of a Russian resolution on 26 March calling for an immediate end to the air strikes.