News of the mission came amid escalating violence in Kosovo itself and a warning from Nato that it has cut the time it needs to unleash air strikes from two days to just a few hours.
In Pristina, the Kosovo capital, four Serb policemen were shot dead after the patrol cars in which they were driving were ambushed. Ethnic Albanians living in the area said they saw four bullet-riddled police cars being driven back from the Grmija district, east of the city centre, followed by two ambulances with lights flashing. A few minutes later, six armoured personnel carriers, carrying masked Serbian police with automatic weapons, were seen travelling at high speed in the reverse direction. Elsewhere in the province, thousands of ethnic Albanians fled their homes as fears grew of a new Serb offensive.
Mr Holbrooke will today fly to Belgrade via Brussels where he will meet Robin Cook, the Foreign Secretary, and his French counterpart, Hubert Vedrine, to discuss the message to be delivered to Belgrade.
The rhetoric from Washington was stepped up yesterday as the US Secretary of State, Madeleine Albright, told Mr Milosevic that he "faces a stark choice: to halt aggression against the Kosovar Albanians and accept an interim settlement with a Nato-led implementation force or bear the full responsibility of Nato military action".
The Prime Minister's office said Tony Blair had spoken to President Clinton and the two had agreed the Kosovo situation was becoming increasingly serious. Both expressed frustration that peace efforts were being obstructed. "They were growing more and more troubled by what was happening on the ground," a spokeswoman said.
Mr Cook warned that Nato's threat of military intervention was "for real", and the Foreign Office said Mr Holbrooke would insist that Mr Milosevic "complies with his undertakings".
However, Germany's Foreign Minister, Joschka Fischer, highlighted reservations among some European nations by striking a more balanced tone, arguing that the West needs "to use every opportunity" to achieve "a peaceful solution and avoid a confrontation". But he added: "At the moment it seems Belgrade is determined to risk that confrontation."
Mr Holbrooke's last-ditch mission was agreed at an emergency meeting of presidential advisers in Washington yesterday after a discussion of the latest Serb offensive. It followed frantic diplomatic efforts to ensure that a Western delegation would be received in Belgrade, and not "snubbed".
Nato is determined not to give the impression that Mr Milosevic has another breathing space. The time necessary for allied aircraft to strike after receiving the order to do so has been reduced from 48 hours to "a matter of just a few hours", an official said yesterday.
In Washington, Sandy Berger, the National Security Adviser, said the Holbrooke visit was a message to Mr Milosevic, "one final time to make clear to him that he faces a very stark choice". And he added: "He can move to the path of peace or he can take this punishment."
Like other US officials he declined to discuss a timetable for bombing, arguing that that would not be "appropriate" but there do appear to be reasons for the White House to hold back. Not only is there opposition in Congress to US embroilment in the Balkans, but Yevgeni Primakov, the Russian Prime Minister, is due to visit Washington tomorrow and Wednesday.
Although billed as a routine meeting, it is considerably more important. The US wishes to build its relations with Mr Primakov to encourage Russian ratification of a pending arms control agreement, and to formulate terms for Moscow's involvement in Nato's 50th anniversary celebrations. With Russia emotionally and diplomatically protective of Serbia, it would be exceptionally difficult for Mr Clinton to order bombing during the Russian premier's visit.
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