Speaking at the Senate Foreign Relations Committee, she referred to signs that Serb forces are preparing for a spring offensive in Kosovo.

Her comments came as the Serbian authorities blocked the return of Kosovo Liberation Army (KLA) representatives after the peace talks in France.

Mrs Albright acknowledged that the Rambouillet talks had not achieved all that she had hoped, but she said that Nato remained ready to act if the Serbs resorted to violence again.

Despite the fact that neither the Serbs nor the ethnic Albanians have fully committed to a political settlement for Kosovo, Mrs Albright said the Yugoslav President, Slobodan Milosevic, remained the primary obstacle to peace.

According to the BBC's correspondent in Washington, Richard Lister, the Clinton administration will have to maximise the pressure on both sides if it hopes to be able to take the plan forward when the two sides meet again next month.

Flight cancelled

Earlier on Wednesday, a special flight intended to return the Kosovo Liberation Army representatives from France to their regional capital, Pristina, was cancelled.

The move mirrors the diplomatic crisis which erupted when the Serbs refused to let them leave Kosovo for the peace conference at the beginning of the month.

The beginning of the Rambouillet talks was delayed until pressure from western diplomats forced the Serbs to reverse their decision.

Peter Biles reports from Kosovo: Darkness has forced the postponement of any travel plans

The Serbian authorities are now reported to be preventing the delegation's return because certain members do not have Yugoslav passports.

Their safety cannot be guaranteed, the Serbs say. The ethnic Albanians are insisting that they will travel together as one delegation and the representatives of the various political parties from Kosovo will not leave members of the KLA stranded in France.

Paul Welsh: Skirmishes continue after peace talks

France is said to have offered the use of a military plane to fly the delegation to Kosovo, but the onset of darkness has now forced the postponement of any travel plans until Thursday.

A BBC correspondent in Kosovo, Peter Biles, says the journey was always likely to be problematic because the delegation includes members of the KLA, which is at war with the Serbian authorities.

Ethnic Albanians to form government

On Wednesday the ethnic Albanians said they had agreed to form a provisional government, with a prime minister nominated by the Kosovo Liberation Army.

But the KLA's political chief Adem Demaci said such a government would have no legitimacy.

"The KLA and its political representative have not been consulted on the formation of a so-called provisional government," Mr Demaci said.

According to the KLA news agency, Kosovapress, the government will have a mandate to govern until elections are held in Kosovo, as promised at the talks in Rambouillet.

The government will include representation from all three ethnic Albanian parties attending the talks.

Correspondents say the move is an effort by the ethnic Albanians to show that they can speak with one voice on any decision on the future of the province.

Act of defiance

It is not yet clear whether the self-declared government will be based in Kosovo itself or be sited abroad, like the existing government in exile.

Bridget Kendall in Rambouillet: 17 days haven't got them very far

BBC analyst Gabriel Partos says that siting the government in Kosovo would be an act of defiance to the Serbian authorities, which would refuse to recognise it.