The move has raised once again the possibility of Nato air strikes against Yugoslavia, as it represents the latest in a series of acts of defiance by the Belgrade government against international demands on Kosovo.

The official Yugolsav news agency, Tanjug, said Mr Walker had exceeded his mandate, was persona non grata, and he was being given 48 hours to leave.

The BBC's Jason Kaye: "Growing tide of condemnation"

Mr Walker, who heads a team of monitors acting on behalf of the Organisation for Security and Co-operation in Europe, had blamed Serb forces for the massacre of more than 40 ethnic Albanians in the Kosovo village of Racak last week.

The United Nations, the Nato allies and Russia have all condemned the massacre in strong terms.

William Walker: Persona non grata in Yugoslavia

The Serbs have denied responsibility for the killings.

Friday saw the start of a new offensive by Serbian troops and police against ethnic Albanians in Kosovo.

BBC Correspondent Jacky Rowland says that Mr Walker's expulsion is typical of the tactics adopted in recent months by the Yugoslav authorities, who seek to divert attention from one crisis by creating a new crisis.

Prosecutor refused entry

Ms Arbour: "I am entitled to access"

The expulsion came at the end of a day in which Yugoslav border officials barred the chief prosecutor from the International War Crimes Tribunal from entering Kosovo.

Chief Prosecutor Louise Arbour was trying to enter Kosovo to investigate Friday's massacre in Racak when she was turned back at the border.

She called it an affront to the international community and said she was determined to pursue her right of access.

The BBC's Ben Brown: "The Serbs have thumbed their noses at the international community"

"I take the position I have a mandate from the [UN] Security Council which entitles me to on-site investigation and I intend to pursue that," Ms Arbour told the BBC.

The chief prosecutor said she was confident that the matter of her access to the massacre site would be pressed by Nato commander General Wesley Clark, who intends to meet Yugoslav President Slobodan Milosevic in Belgrade.

Fears of cover-up

Serbian police are being blamed for the massacre

Meanwhile there are concerns that the refusal of access to Ms Arbour will allow the destruction of evidence which could point to those responsible for the massacre.

Already, Serbian investigators have taken the bodies of the massacre victims from the mosque in Racak, where they had been laid to await burial, to Pristina - the capital of Kosovo.

Nato spokesman James Shea: Walker's expulsion "an outrageous act"

Our correspondent says the removal of the bodies from Racak will cause further anger in an ethnic Albanian community already outraged and traumatised by the worst single act of violence since the conflict here flared almost a year ago.