Mr Blair said there would be no compromise with the Serbian leader, who has offered the alliance a unilateral ceasefire.

The prime minister also insisted there were no splits in the Nato alliance.

He said: "In the next few days, Milosevic is going to be trying everything he possibly can to split the allies apart from each other - negotiating ploys, half-hearted peace attempts.

Tony Blair: Reverse ethnic cleansing

"Only one thing will do: that he gets his forces out of Kosovo, he stops the ethnic cleansing, that policy is reversed, the international force comes in and the Kosovars return to their homes."

The policy of ethnic cleansing had to be "reversed and be seen to be defeated" before the bombing campaign would stop, Mr Blair said.

The prime minister spoke hours after Foreign Secretary Robin Cook said Britain was "ramping up" its forces against Serbia.

The BBC's George Eykyn: The Harriers used devastating cluster bombs

RAF Harriers have for the first time attacked Serbian army and special police forces accused of carrying out atrocities in Kosovo.

The 10 UK aircraft dropped cluster bombs on Serb tanks and vehicle convoys on the 14th night of Nato bombing.

'Appalling acts'

Mr Blair again insisted Nato's action had not caused the refugee crisis.

Some 300,000 people were displaced from Kosovo last year, he said.

"True it is that over these past two weeks, as this campaign of ethnic cleansing has been stepped up, as these appalling acts have taken place, he has been doing more and more and more.

Robin Cook: Dismissed the Serb ceasefire as a "sham"

"But don't let anyone for a moment think that if Nato hadn't taken its action he would have been sitting there with his 40,000 troops and 300 tanks doing nothing.

"Don't let anyone run away with this idea that if Nato hadn't taken the action there would be peace in Kosovo. There wouldn't. There would be ethnic cleansing but absolutely nothing done to stop it."

Mr Blair said Britain was prepared to play its part in shouldering the burden of the refugee crisis, but it was important that President Milosevic must realise his policy of ethnic cleansing would be reversed.

'Sham' ceasefire

Earlier, the foreign secretary dismissed a unilateral Serb ceasefire by President Milosevic as a "sham".

Mr Cook said: "It must have dawned on him that Nato can keep going longer than him. He knows he cannot win by military conflict."

The foreign secretary said he had received reports of a number of new massacres in the past 24 hours.

"These are based on reports from within Kosovo," he said.

"There is of course one very easy way for Belgrade disprove these reports.

Ibrahim Rugova seen last week in talks with Slobodan Milosevic

"That is to accept my proposal to allow an independent war crimes tribunal in to the area."

The foreign secretary again promised those responsible for orchestrating war crimes, including rape and mass murder, would be brought to justice however long it took.

Mr Cook also issued two further demands to the Serb leader, challenging him to free the moderate Albanian Kosovan Dr Ibrahim Rugova, who is holding talks with Serb leaders.

"Nobody will believe that those negotiations are for real while Dr Rugova is kept under duress," Mr Cook said.

The Serb leader should also tell the truth to his people about what had happened to the country's wealth, he added.