Paula Ghedini, a UNHCR spokeswoman, said camps in the former Yugoslav republic were full and aid workers at Lojani had been prevented from taking supplies to thousands of refugees being kept on the border by the Macedonian authorities.

A further 3,000 people are reported to be stuck in freezing conditions in the hills on the border at Malina Mali.

This latest call for help comes amid growing concern for Kosovo-Albanian refugees in Montenegro following the reported killing of six people on Monday by Yugoslav soldiers.

Exodus

The UNHCR said it had made it clear to donor and refugee host countries that it wanted larger numbers of people taken out of Macedonia, which is currently hosting some 130,000 refugees.

Spokesman Kris Janowski said: "It's an exodus which is potentially destabilising for the region and huge international solidarity and co-operation is needed to deal with it and keep a lid on it."

Claire Doole reports: The plight of Kosovo's refugees remains critical

Macedonia's president has described the influx into his country as the greatest threat to Macedonian security since independence in 1991.

"I believe dangers for Macedonia are growing. Macedonia's state interest is to preserve internal stability and external security," said President Kiro Gligorov.

He said the flood of Kosovo Albanians threatened to upset his country's fragile ethnic balance. Correspondents say that Macedonia already has a sizeable Albanian minority.

Nato struck President Milosevic's party HQ

The BBC correspondent in the capital Skopje, Paul Wood, says the situation in Macedonia is urgent as UN sources report that as many as 50,000 more refugees could be on their way from one of Kosovo's biggest towns, Gnjilane, which previously had not seen any large movement of people.

The report cannot be independently confirmed, but international officials in Macedonia believe it could be evidence of a final push by the Serbian authorities to empty Kosovo of its Albanian population, our correspondent says.

John Simpson: "Nato took the war to President Milosevic himself"

European countries have offered to take in 70,000 Kosovo refugees, while so far only around 17,000 have been flown out of Macedonia.

"We're not asking for offers, we're saying more people have to be taken out," said Mr Janowski.

However, large-scale evacuations from the small airport in Skopje were not easy because of the heavy numbers of relief flights, and host countries might have to to use bigger aircraft to intensify the evacuation, he said.

Alarm in Montenegro

While more refugees are expected to arrive in Macedonia, the UN says that refugees who had fled to Yugoslavia's junior republic, Montenegro, are leaving on buses for Bosnia-Herzegovina and Albania.

Brian Barron reports: Montenegro is immensely vulnerable to pressure from Belgrade

This exodus follows an incident near the village of Rozaje on Monday, in which drunken army reservists killed six people, including both locals and refugees.

The UNHCR said: "Alarm has spread throughout Rozaje and many Kosovars, as well as some locals, have been leaving the area."

There have been reports of local Muslims leaving Rozaje on four buses bound for Sarajevo, while UNHCR staff saw 30 buses leaving the town, the refugee agency said.

However, the Yugoslav 2nd Army command gave a different version of events, saying in a communique that one of its units had come under fire from "a large group of terrorists of the Kosovo Liberation Army" and in a "resolute action" had broken them, killing four.

According to the UNHCR, some 70,000 Kosovo Albanians have fled to Montenegro, but several hundred pour out again each day towards Albania.

Helicopters signal new phase

As the refugee crisis showed no signs of abating, Nato embarked upon the fifth week of its air campaign on Yugoslavia.

US attack helicopters have arrived in Albania, marking a new phase in Nato's military campaign against Yugoslav forces.

Armed with laser-guided missiles, Apache helicopters can be used against artillery, tanks and troop concentrations.

Unlike Nato jets, they operate at low levels, within range of Yugoslav weapons.

(Click here for a map showing latest strikes)

The alliance struck a 24-storey building in the Yugoslav capital housing the headquarters of President Slobodan Milosevic's Socialist Party of Serbia.

The BBC's Jim Fish: "A blow to the Milosevic family"

Nato spokesman Jamie Shea said the tower block was "a high-value target at the very centre of the power structure in Belgrade".

Dr Shea said the building was the centre of the Yugoslav propaganda machine, and an important link in the defence communication network.

There were also reports of Nato strikes on northern and central Serbia, including the last remaining bridge over the Danube in the town of Novi Sad. An oil refinery there was also reported hit, as was a TV transmission tower outside the city.

Nato says the number of targets being hit each night in its air strikes has doubled since the campaign began.

• 'Ground troops an option' - Blair
• 'Trapped' in Kosovo
• Apache: the all-weather attacker
• US volunteers join Kosovo rebels
• In the belly of a B-52