The rival Serb and ethnic Albanian delegations have been meeting over the last week at a chateau outside Paris to try to hammer out a peace deal over the province.

Dr Solana told reporters in Canada: "At this point, we are having very, very, very, very serious difficulties with the Serb party."

Diplomatic Correspondent Barnaby Mason reports: Little evidence of progress at the talks

Earlier in the day, Serbian President Milan Milutinovic said that his delegation was prepared to hold face-to-face talks with the rival ethnic Albanian side.

But he insisted there could be no full negotiations unless ethnic Albanians signed a disputed declaration of principles which would protect Yugoslavia's sovereignty over Kosovo.

The Albanian delegation insists on the Serbs agreeing to an interim settlement with a guaranteed referendum on independence.

Mr Milutinovic denied the Serbs were holding up progress at the peace talks by demanding both delegations sign the declaration.

Milan Milutinovic: Insisted Serbs were not wasting time

Nato has threatened air strikes against Yugoslav targets if Belgrade jeopardises the negotiations aimed at bringing the conflict in the province to an end.

Dr Solana added: "I have to tell you that Nato is prepared to use force in order to have ... stability and peace, enduring peace, lasting peace, in that part of Europe."

Mr Milutinovic, a close ally of Yugoslav President Slobodan Milosevic, warned that any Nato attack would prove disastrous.

"I cannot believe that they want to have a Vietnam in Europe. That would literally mean blood up to the knees," he said.

Albright flies in

US envoy Christopher Hill, the talks' chief mediator, said that he was set to present further segments of an international peace plan to the two sides.

But he admitted there was still no deal on initial political proposals handed out last Sunday for a three-year interim period of autonomy for the disputed province.

The BBC's Paul Royall: "Talks are still struggling"

United States Secretary of State Madeleine Albright was expected to throw her political weight behind the talks with a visit to Rambouillet due on Sunday. US Government officials said Ms Albright was going there to "conduct her own on-the-ground assessment" of the negotiations.

European Union foreign ministers were also due to meet on Sunday in Paris to discuss the situation, while ministers from the six-nation Contact Group on the former Yugoslavia might convene a meeting on the same day.

Albanian delegates seek a referendum on independence for Kosovo

The Contact Group, made up of the US, the UK, France, Italy, Germany and Russia, has the task of deciding whether the talks between the rival delegations should continue next week.

In preparation for a possible settlement between the Serbs and ethnic Albanians, Nato allies are moving ahead with plans for a peacekeeping force in Kosovo.

A Pentagon official announced on Friday that about 2,200 US Marines would be sent to Kosovo.

And British tanks and artillery will be sent to the Balkans next week.