It said there was clear evidence that Serb forces massacred a large group of men in the village of Meja in south-west Kosovo.

The reports emerged as the Russian special envoy to Yugoslavia, Viktor Chernomyrdin, held five hours of talks with President Milosevic during his latest mission to try to resolve the conflict.

Mr Chernomyrdin is quoted as saying that the meeting had made solid progress. He said he would be visiting London and Paris in the next few days.

(Click here to see a map of alleged Serb atrocities)

UN High Commissioner for Refugees (UNHCR) spokeswoman Lyndall Sachs said waves of refugees had spoken about bodies lying in ditches and in fields near Meja.

"I think it is pretty clear that the Yugoslav authorities are really stepping up their campaign of forciblle deportation," she told the BBC.

Jeremy Bowen in Albania: People talk of systematic slaughter

A BBC investigation - using accounts of refugees crossing into northern Albania - suggests that hundreds of men were killed near Meja.

"Those we spoke to said the men were led away into a field in a small valley. There they were made to sit down, around 300 men in all.

Lyndall Sachs of the UNHCR: They have all spoken about bodies lying in ditches

"One eyewitness told the BBC that the men were all forced to raise their hands and salute Serbia and say: 'Long live Slobodan Milosevic!' The women and children left in the convoy were then forced to drive on."

He said one woman reported hearing sustained gunfire. Other refugees spoke of seeing a huge pile of bodies in a field.

Russian diplomacy

Mr Chernomyrdin's talks with the Yugoslav president followed meetings on Thursday in Italy and Germany, where all sides reported movement in the search for an agreement.

Speaking to reporters before returning to Moscow, Mr Chernomyrdin said he had discussed sending some kind of international force into Kosovo under the control of the United Nations.

However he declined to say if the force under discussion would be armed or if it would include Nato troops.

Last time, the two sides came away with different views on what had been agreed.

Also in Belgrade is the Reverend Jesse Jackson, who has met and prayed with the three captured US soldiers.

He said they were in good health but their early release has been ruled out by a Yugoslav official who said it was "not on the agenda".

Nato blitz on Belgrade

John Simpson: The bombing of Belgrade is intensifying

The diplomatic initiative followed the most intense night of bombing raids on the Yugoslav capital, Belgrade, since the start of Operation Allied Force.

Nato targeted the headquarters of the army and special police and the Ministry of Defence buildng in Belgrade.

Three airfields, a television transmitter south of the capital and three radio relay stations were also attacked by successive waves of Nato bombings.

Mike Williams: Very heavy bombardment

"Yesterday was the most intense day thus far in our air operations thus far over Yugoslavia," Nato spokesman Jamie Shea said on Friday.

"We hit a very impressive range of targets, both at a strategic and a practical level."

Further assaults were carried out against Serb forces in Kosovo and the Novi Sad oil refinery in the north.

Nato admitted one missile hit a residential area. Local reports said two civilians and a policeman were killed in the attacks.

The area was also been struck by an earthquake. The tremor shook buildings in the centre of Belgrade, although reports said damage was not serious.

Earlier, Serbian state television, RTS, went off the air in the middle of its late-night evening news bulletin, following a Nato strike on its main transmitter.

(Click here to see a map of recent Nato strikes)

Oil embargo

There are likely to be further disagreements over Nato's planned oil embargo against Yugoslavia which is due to be put into effect on Friday.

Speaking at the Pentagon, American Defence Secretary William Cohen said the US was in favour of using force if necessary to stop supplies getting through to Serbia.

However, France says stopping and searching ships would require a United Nations mandate.

Correspondents also point out that Russia has failed to agree to adhere to an embargo.

Bulgarian Ambassador to the UK Valentin Debrov: "Nato needs our support"

Meanwhile the Bulgarian Parliament was due to begin a debate on Friday on whether to ratify the government's decision to allow Nato to use some of its airspace for Operation Allied Force.

The discussions follow Nato's admission that one of its missiles had accidentally hit a suburb of the Bulgarian capital, Sofia, on Thursday.

Other top stories

• UN rights chief slams Nato bombings
• Refugees speak of slaugher at Meja
• Jesse Jackson prays with POWs
• Belgrade under fire - in pictures
• US warns against defying oil embargo

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