GENEVA (Reuters) - The United Nations accused Belgrade Friday of increasing the brutality and scale of ethnic cleansing in Kosovo which NATO has vowed to halt with its air war.

A train disgorged some 3,000 ethnic Albanians at the Yugoslav-Macedonian border Friday. They walked past minefields through a no man's land where some 45,000 earlier refugees were stuck for days in filth and squalor as Macedonia hesitated to let them in.

In Albania, officials said about 2,500 refugees forced from their homes by Serb soldiers had streamed into the north of the country and many more were headed across the border.

International monitors said heavy shelling was heard in the remote Tropoje district of northern Albania near the Yugoslav frontier as Albanian police reported fighting at a nearby border post.

NATO kept up its air attacks on Yugoslavia, bombing targets around Belgrade overnight despite admitting it might have mistakenly hit civilians in Kosovo.

NATO officials were expected to face tough questioning at a regular news briefing in Brussels to clear confusion still surrounding Wednesday's attack.

The alliance denied a Yugoslav news agency report that NATO missiles had struck a refugee center overnight in the Serbian town of Paracin. The official Tanjug agency said the refugees were in shelters during the raid and escaped injury.

At NATO military headquarters in Mons, Belgium, an official said alliance aircraft had struck an ammunition dump in the vicinity of Paracin and a radio relay station 30 km (20 miles) away. In both cases NATO was confident it had hit only the military targets, the official said.

The air raids underscored NATO's vow not to let what it called the mistaken bombing of a civilian vehicle two days ago weaken its resolve to cripple President Slobodan Milosevic's war machine, largely by targeting fuel supplies and communications.

The Geneva-based United Nations refugee agency, UNHCR, accused Milosevic of wanting to empty Kosovo of those who remained of the original 1.8 million ethnic Albanians.

``The expulsions which were put on hold or slowed down over the last two weeks have now resumed with full force,'' UNHCR spokesman Kris Janowski told a news briefing. ``The effort by the Serb authorities to expel the entire ethnic population of Kosovo is again under way.''

Citing testimony from the latest batch of refugees to leave the Serbian province, he said: ``We can tell that terrible things are happening in Kosovo.''

He added: ``The brutality of the expulsions as well as the scale of the expulsions is picking up.''

Janowski said UNHCR, the U.N. High Commissioner for Refugees, did not see how ethnic Albanians inside Kosovo could be helped as long as Yugoslav forces remained in the province.

NATO has ruled out suspending its unprecedented air campaign until Milosevic withdraws his troops from Kosovo.

The New York Times reported Friday that the Pentagon intended to ask President Clinton to activate as many as 33,000 reservists and National Guard troops to strengthen the attack on Yugoslavia.

The request, expected before Monday, follows a week in which NATO has intensified its air raids and asked for an extra 300 aircraft from the United States, bringing its total air armada to 1,100.

Belgrade's sky was lit up by anti-aircraft fire as NATO planes flew over the city overnight.

Tanjug said the city's suburb of Rakovica was hit by two missiles and it also reported an attack on Belgrade's Pancevo oil refinery.

``Installations on the Pancevo oil refinery and an oil depot in the nitrogen plant were hit,'' Tanjug said.

For the first time, NATO hit targets near the Hungarian border. Four explosions were reported in the northern Serbian town of Subotica, just 12 km (eight miles) from Hungary.

NATO admitted Thursday it mistakenly bombed a civilian vehicle in a convoy in Kosovo but said ``one tragic accident'' would not weaken its resolve to forge ahead with its air war.

Serbian state television said Friday Yugoslavia had asked for a meeting of the U.N. Security Council to condemn the attack on the convoy.

The United States said the mistaken attack, which Serb media said killed 64 people, was the regrettable result of an air war provoked by Milosevic.

Correspondents taken to the scene saw a man's body seated at the wheel of his tractor, with two severed legs on the trailer behind.

On the grass nearby lay a man's head and about a dozen bodies, clearly mutilated by explosions.

``There were four attacks, one after another. Seventy-two people were killed and dozens wounded,'' said Colonel Slobodan Stojanovic, a Yugoslav army spokesman accompanying the group.

``Some were severely wounded and more people will die. It was a deliberate act to create as many civilian casualties as possible,'' Stojanovic said.

In Moscow, the State Duma lower house of parliament voted in favor of letting Yugoslavia join the loose Russia-Belarus union, passing a non-binding document aimed at prompting the Russian government into action.

President Boris Yeltsin said a week ago that he favored bringing Yugoslavia, a fellow Slav and Orthodox Christian state, into Russia's union with its ex-Soviet neighbor.

But Thursday Yeltsin's deputy chief of staff played down the possibility of a union.