In a Downing Street address, he said war could sometimes be necessary to uphold civilised values.

He spoke minutes after Nato Secretary General Javier Solana announced air strikes on Yugoslavia had been suspended after Serbia complied with the allies' five key demands.

The refusal of Serb President Slobodan Milosevic to hold serious negotiations earlier had forced the allies to begin the conflict, Mr Blair said.

Tony Blair: "Nato will remain vigilant"

"Milosevic now knows and the world now knows that we will not let racial genocide go on unchallenged."

The world had become familiar with the term "ethnic cleansing", but it was important not to forget the reality behind it.

"What it meant in reality for hundreds of thousand of innocent people in Kosovo was murder and rape, brutality and barbarism, on a scale we thought our continent had seen the end of.

Tony Blair: "The world now knows we will not tolerate racial genocide"

"Milosevic has now given his word that he will withdraw his force. He has started his withdrawal, but as you know the Balkans is littered with his broken promises."

Nato's challenge now lay in ensuring the safe return of the refugees to their former homes, the prime minister said.

"We have made a pledge to the refugees that they will go home and live in peace. We intend to honour that pledge."

Security vacuum fear

Robin Cook: We don't want a security vacuum

A full withdrawal of Yugoslav troops has already begun from Kosovo.

UK forces will be among the first to cross the border into Kosovo as part of the deployment of the international force which will return the refugees home.

Robin Cook: A phased programme will follow troop withdrawal

Foreign Secretary Robin Cook said earlier: "We are very keen that the moment the Serb forces are withdrawing from the sector opposite us across the border in Macedonia that we start to move in.

"The reason for that is we don't want a security vacuum.

"We want our forces to go in there to secure the peace, secure the territory and start to create the conditions in which the refugees can go back so that there is no gap between one force leaving and one force going in."

The possibility of a Nato military campaign against Serbia would remain until the last of the troops had left Kosovo, he warned.

Mr Cook said Nato would be alert for any "trick" President Milosevic could pull, but said the Serb leader was now in a much weaker position.

At the Ministry of Defence briefing Defence Secretary George Robinson said the agreement was a victory Nato could be proud of.

He said: "I believe Nato is on the way to accomplishing a great deed.

"The alliance has been prepared to go out of its way to fight for a downtrodden people against a campaign of ethnic hatred and it has won, and that, I think, is something we can all be proud of at the end of what has been a dark and difficult 20th century.