Nato's Generals will take a tough message to Belgrade. They will make clear that the option for air strikes against Serbia remains open and that President Milosevic will avoid them only if he complies fully with existing United Nations resolutions calling for an end to violence.

Nato is also demanding that President Milosevic grant immediate access to the scene of last Friday's massacre of Kosovar Albanians to the chief prosecutor of the International War Crimes Tribunal, Louise Arbour, and a team of Finnish forensic experts.

Nato's Secretary-general, Javier Solana, condemned the massacre and said Mr Milosevic bore personal responsibility for the behaviour of his security forces.

"This represents a flagrant violation of international humanitarian law," he said in his statement, which was read out by his spokesman Jamie Shea.

Nato wants the Yugoslav government to help bring those responsible for the massacre in the village of Racak to justice, but that may be a vain hope given that the lines of command may lead up to Mr Milosevic himself.

Serbian non-compliance

The generals will appeal both to the Serbs and to the Kosovar Albanians to refrain from further military action. But it is clear that the warning is directed principally against the Serbs.

Mr Milosevic will be told he is in very clear non-compliance with the cease-fire agreement reached last October.

Mr Milosevic has tested Nato's resolve many times

Sunday's meeting of ambassadors from the 16 Nato countries put the prospect of Nato air strikes onto the back burner. But the so-called activation order remains in force.

This means, if the generals' mission to Belgrade fails, the 700 civilian cease-fire monitors in Kosovo could be evacuated to prepare the way for military force to be used as soon as a political decision is taken.

Past ultimatums ignored

Nato believes that autonomy for Kosovo within Serbia remains the only long-term solution to the conflict.

However, the ethnic Albanians, who make up 90% of Kosovo's population, are growing more and more committed to full independence the longer Serb aggression continues.

Nato's credibility is on the line now. Mr Milosevic has agreed to Western ultimatums on several occasions before only to return to violence later.