BBC Correspondent Paul Wood: "Refugees continue to leave."

His remarks came as BBC correspondents reported that thousands of ethnic Albanian refugees continued to flee over the border to Albania to avoid a Serb offensive in the province.

"The full power of Nato is considering all options, including the most radical," Mr Robertson told BBC Radio 4's Today programme.

"We are examining very carefully military options that could and might have to be made available."

George Robertson: "Military options not ruled out."

Britain has called an emergency meeting of the Contact Group of major powers in London on June 12 to discuss the escalating fighting and refugee exodus.

Albania, which has received thousands of ethnic Albanians from Kosovo, has called for international action to stop an open war in the province.

Nato defence ministers are also meeting next week in Brussels to consider the crisis.

Mr Robertson said Kosovo would be at the top of the agenda at the Nato meeting but declined to describe the military options under consideration.

Ethnic Albanian refugees arrive in Albania after fleeing the fighting

However, he said the Yugoslav President Slobodan Milosevic "should be certainly under no illusion at all that these options are being considered, are being planned, are being looked at.

"We are giving the clearest possible signals through political, military and economic pressure that Mr Milosevic must stop and think, must look at other alternatives, and look for a peaceful and political solution to the problems that exist."

Strong signals

BBC Correspondent Jon Devitt reports from Kosovo-Albania border

The Contact Group comprises the United States, Britain, France , Germany, Russia and Italy.

The UK Prime Minister, Tony Blair, says the world needs to send the "strongest possible signal" to the warring parties.

The ethnic Albanian leaders pulled out of talks, scheduled for Friday, with the Serbian Government in Belgrade, in protest at Serbian military operations to reassert control of roads and towns along the Albanian border.

Nato spokesman Jamie Shea: "NATO has learned the Bosnia lessons rather well"

Last month, the Contact Group lifted some sanctions against Belgrade after President Milosevic agreed to regular meetings with Kosovo's ethnic Albanians to try and broker a peace deal for the province.

But the co-ordinator of the Kosovo Albanian team, picked recently to negotiate with the Serbs, said it was impossible for any meetings to be held.

He said negotiations could only resume once the fighting ends and police allow people back into regions in Kosovo that they have sealed off.

Massive operation

Many people have abandoned their homes

Over the last seven days the Serbian police are reported to have mounted a massive operation to get rid of alleged separatist guerrillas of the Kosovo Liberation Army (KLA) in western Kosovo.

They have attacked several villages that lie close to the border with Albania.

Eyewitnesses who escaped from the region talk of houses being burned and shelled.

The BBC correspondent in the region says the Serbian authorities seem to be determined now to try to eradicate the KLA but the ethnic Albanians say civilians are being killed.

Nato believes Serb forces are trying to cut-off the guerrillas from their safe haven and source of supplies in Albania by clearing and occupying a swathe of land on the border.

Refugee exodus

The United Nations refugee agency says more than 40,000 ethnic Albanians from Kosovo have fled their homes in an attempt to escape the latest security operation.

More than 10,000 are reported to have crossed the border into Albania. But UN officials believe that figure will double.