The warning came as Russia mounted an effort to keep Belgrade out of Nato's sights.
Igor Ivanov, the Russian Foreign Minister, and Igor Sergeyev, the Defence Minister, met President Slobodan Milosevicof Yugoslavia and Milan Milutinovic, the President of Serbia, in a last-ditch effort to head off a conflict.
Yugoslav authorities have put the country's air defences on high alert, bracing for the possibility that Nato may finally fulfil oft-repeated threats to hit Serb targets. Klaus Kinkel, the German Foreign Minister, said last week the clock "is at one minute to 12".
Kofi Annan, the United Nations Secretary General, will deliver an assessment today of whether Serbia is complying with demands from the international community in the province. If he takes a negative view, that would give the green light for Nato to accelerate its planning for air strikes.
Reports over the weekend said that Britain was already putting Royal Marines on stand-by to go to Kosovo, but George Robertson, the Secretary of State for Defence, refused to comment. "Our plans are not going to be disclosed to Slobodan Milosevic on any programme," he told the BBC yesterday. "But I think you can take it that a whole series of options have been worked through and that we are ready to move on them." The Cabinet meets today to approve formally the use of force.
Nato planning is far advanced, and defence ministers may hold a special meeting on Wednesday, as the US Defense Secretary, William Cohen, is passing through Europe on his way to Asia. A special meeting of Nato ambassadors will also be held today.
Plans for attack have run into criticism from conservative American politicians who fear the attacks are planned just to help President Bill Clinton. Trent Lott, the leader of Republicans in the Senate, said he was deeply sceptical that in "what appears will be three weeks before an election, we are going to go in and bomb some artillery tubes inside Kosovo". But others from Mr Clinton's own party were more supportive, and any operation would probably have bipartisan support. If a diplomatic solution includes "putting a few more people on the ground from Nato to make sure that the peace is kept, then I'm for it", the House Democratic leader, Dick Gephardt, said.
Serbia has made some efforts to assuage the West, installing an interim government that includes ethnic Albanians and staging a limited military pull-out. But without further action, this will not be enough.
Western opinion has been further outraged by a series of claims of massacres by Serb troops. Human Rights Watch, the US-based organisation, reported yesterday that both Mr Milosevic's government and the ethnic Albanian rebels have carried out atrocities since the inception of the conflict. But it said the government's crimes were on a much larger scale.Reuse content