The UN refugee agency says the move is a measure of last resort but would have to be taken within the next few days.

The agency is also due to begin evacuation flights to Canada, the United States and Australia as part of the effort to make space for the continuing flood of refugees.

Weight of numbers

Aid agencies had hoped to keep all the refugees within Europe but sheer weight of numbers forced them to change their plans.

On Tuesday, the UK government announced it would be taking around 1,000 refugees per week.

Fergal Keane in Albania: The refugees described a journey out of hell

Around 11,000 Kosovo Albanians arrived in Macedonia on Monday - the biggest influx in weeks in what correspondents say appears to be a Serb attempt to destabilise the republic.

In total, more than 675,000 people have fled Kosovo since Nato began its air campaign against Yugoslavia on March 24.

Refugees register for transportation to the United States

Existing camps in Macedonia are already overcrowded and insanitary, and the Macedonian government is refusing to build more.

With the international airlift to countries further afield bogged down in bureaucracy, the UN says in the next few days refugees will have to move on to Albania, but a spokesman in Geneva insisted that the evacuation would be voluntary.

Camps being built there by Nato troops have capacity for another 6,000 refugees.

Ethnic balance

Massive tent cities are already severely overcrowded

The UN wants to do everything possible to ensure Macedonia's goodwill and allay fears that the influx of Kosovo Albanians will disrupt the delicate ethnic balance in the republic.

Since the crisis began, Macedonia has taken in 200,000 refugees. But numbers have risen considerably in the past weeks, with Serb soldiers forcing people into Macedonia and away from Albania to try to destablise the country.

The UN describes the logistical situation in Albania as a nightmare, but says at least there is the political will to take in the refugees.

Macedonia's fragile goverment

Paul Wood in Skopje: Diplomats are worried about Macedonia's ability to survive as more refugees flood in

It has emerged that the Macedonian Government came close to falling two weeks ago when the ethnic Albanian minority party threatened to walk out.

The party's leader said he made the threat because of what he said was poor treatment of Kosovar refugees by the mainly Slav security forces, but said he backed down because he did not want to distract Nato with another Balkan crisis.

However, the BBC's correspondent in Skopje says tensions remain and many Western diplomats there are worried about the ability of the country to survive as more refugees flood across the border from Kosovo.

Aid appeal

The UN food agency has appealed for $9m in emergency aid to help poor farmers in both countries who have taken in refugee families from Kosovo and whose livelihood is threatened by the conflict.

On Tuesday, the agency received what it said was its largest ever private donation from an Argentine businesswoman.