Mr Blair met US senators on Capitol Hill after a two-hour strategy meeting with President Bill Clinton in the White House.

He insisted no change had occurred on the Alliance's attitude towards sending a ground force into Kosovo.

"The position in relation to ground troops was set out by the secretary general of Nato yesterday and our objectives for this conflict are very very clear," he said, ahead of meeting the senators.

"They were set out in the beginning, they have been repeated all the way through and they are quite simply that Milosevic must get his forces and his paramilitaries out of Kosovo.

"We must have an international military force that goes in to allow these people to go home."

UK Foreign Secretary Robin Cook, speaking at a Washington news conference, added: "We are not going to commit ground forces in a hostile environment, nor do we need to. Time is our greatest ally."

Standing alongside him, US Secretary of State Madeleine Albright said while no plans existed to send in ground troops it was "prudent to update our plans".

'No Milosevic veto'

The BBC's Nicholas Witchell: "It has always been the intention to send ground troops in"

Earlier, Mr Clinton and Mr Blair held their first meeting since Nato air strikes began in March, in which they discussed the "next step".

It came after Mr Blair told the House of Commons that ground troops remained "an option" - a remark seen by many as a shift after previous statements ruling out their deployment.

Shadow Foreign Secretary Michael Howard insisted Mr Blair had effectively given himself "carte blanche" to deploy ground forces.

He said: "We would say this is a very significant shift and it must be explained.

"It's a shift that may be justified but we need a full explanation."

But UK Armed Forces Minister Doug Henderson said any decision on the deployment of ground forces in Kosovo would not be taken at Nato's 50th anniversary summit in Washington this weekend.

Speaking at the daily Ministry of Defence briefing on Thursday, Mr Henderson said such decision-making was carried out by the North Atlantic Council of Nato ambassadors in Brussels.

John Maples and Donald Anderson discuss the change in government attitude

"Can I make it clear in relation to ground forces that, as the prime minister said yesterday, the difficulties of a land force invasion of Kosovo against an undegraded Serb military regime are formidable but all options are always kept under review.

"Can I also make it clear that President Milosevic does not have a veto on when or where military action will take place.

"This is precisely our position. Nothing more, nothing less."

Appeal to Russia

Ground troops "an option", says Mr Blair

In the US, British officials played down any talk of a shift in strategy but said another four or five weeks of bombing in Yugoslavia might allow a decision on the "next step".

A Downing Street spokesman said Mr Blair and Mr Clinton agreed that Nato "would show the world the strength of the alliance's resolve to defeat ethnic cleansing in Kosovo".

A White House spokesman said they also discussed ways to tighten the "economic noose" on Yugoslavia, in particular limiting imports of petrol and oil.