Mr Blair discussed the situation for around half an hour with the Russian leader. The prime minister spokesman said he urged Mr Yeltsin to use his influence with Yugoslav President Slobodan Milosevic to stop the "barbarism".

A Kremlin statement said the two leaders agreed international efforts to halt the crisis should not lead to violation of Yugoslavia's territorial integrity.

In the second 30-minute conversation, Mr Clinton agreed the major powers should issue a clear condemnation of the actions of the Yugoslav forces.

UK Foreign Secretary Robin Cook also made it clear Britain is urgently discussing how to tackle the worsening conflict in Kosovo with its allies.

In a speech to a conference on the EU at the London School of Economics, Mr Cook said 50,000 people from the province's Albanian majority had been forced to flee an offensive by Serbian police battling guerrillas of the Kosovo Liberation Army last week.

The fighting had cost up to 300 lives in recent weeks, he said.

'Milosevic has crossed the line'

Watch Peter Biles' report from the refugee trail out of Kosovo

Mr Cook told the conference: "President Milosevic over the past week has crossed the threshold.

"The use of tanks, of artillery, of the might of the military army against civilian centres of population, is wholly unacceptable within the modern Europe."

The foreign secretary said the EU could not tolerate a confrontation that was so explicitly ethnic in its motivation and in its objectives.

Robin Cook will chair a meeting about Kosovo on Friday

Mr Cook is to chair a meeting of the EU's general affairs council in Luxembourg on Monday when Kosovo will top the agenda, as well as a Kosovo summit of the contact group of six major powers in London on Friday.

He said the UK, which currently holds the rotating presidency of the EU, was consulting its allies about the wording of a resolution for the United Nations Security Council to authorise necessary measures against Belgrade.

Mr Cook said: "We are examining all options and ways that might be helpful to de-escalate the violence and producing a stabilisation within Kosovo.

"Peace within Kosovo will also, to be stable and permanent, require democracy in Belgrade.

"I firmly believe that peace and democracy and reconciliation go hand in hand.

Threats to freedom

"I think it is therefore important that the international community does not just respond to Kosovo as a military challenge but also respond to the threat to democracy and freedom of expression within the Federal Republic of Yugoslavia, particularly in Serbia."

Mr Blair is reported to have warned that British troops may be needed in the province.

Military intervention inside Kosovo is out of the question because the UN Security Council would not allow intervention inside a member state.