Yugoslav reports say Nato's 26th night of bombing caused immense damage in the central town of Paracin and largely destroyed a government building in the centre of Yugoslavia's second city, Novi Sad.

The official news agency Tanjug says repeated bombing earlier of the main oil refinery at Pancevo near Belgrade polluted the River Danube with an oil slick.

Only favourable winds stopped toxic gas from a chemical plant bombed nearby causing severe health problems, it said.

(Click here for a map showing recent Nato strikes)

The BBC's Nicholas Witchell: A fog of war surrounds last week's attack on refugees

The US ambassador-at-large for war crimes, David Scheffer, told American television that the scale of Serb atrocities in Kosovo might be much greater than previously thought.

"We have upwards of 100,000 men that we cannot account for," in Kosovo, he said.

Many Kosovar Albanian men had just disappeared: "We have no idea where they are now."

He raised the prospect, as have other western leaders, of an indictment by the War Crimes Tribunal of Yugoslav President Slobodan Milosevic.

Meanwhile, UK Prime Minister Tony Blair and American officials continued to insist that the air campaign was working.

"The most important thing now, particularly in advance of the Nato summit (later this week) is to show that there is an allied will stronger than that of Milosevic to make sure that every single one of Nato's demands - just demands - are met and are met in full," he told American television.

Confusion over convoy bombing

As questions remain over the attack last week on a civilian convoy in Kosovo, the Pentagon has confirmed that a recording purporting to be a Nato pilot had nothing to do with the incident.

Nato spokesman Jamie Shea: "Nato would never knowingly mislead any journalist"

The bombing killed more than 70 Kosovo Albanian refugees near the town of Djakovica.

Last Thursday, Nato released a tape in which a Nato pilot appeared to say he had attacked only Yugoslav forces.

Asked about the audio tape confusion, the UK Defence Secretary George Robertson and NATO spokesman Jamie Shea both said some confusion was inevitable in times of conflict.

"I think it's been a confusing incident, regrettably," Jamie Shea told BBC radio.

Defence Secretary George Robertson takes questions on the convoy bombing incident

And the controversy has been further complicated by a report that a British pilot warned the American Nato pilot who launched the attack.

According to the Daily Express newspaper, a British Harrier GR-7 pilot warned the American pilot that his target was largely civilian, but the US pilot went ahead with the bombing.

At a defence briefing on Monday, George Robertson said that he could not confirm any details of the Daily Express story, but the incident was under investigation.

"I'm not going to add to the speculation here ... There will be no cover-up," said Mr Robertson.

Refugees flood in

Nato's campaign appears to be having no impact on the numbers of refugees flooding out of Kosovo.

Aid workers say 40,000 arrived at border crossings over the weekend.

And there is renewed concern for refugees at the main crossing into Albania, which was closed late on Sunday after Yugoslavia severed diplomatic relations, accusing Albania of aggression.

The Yugoslav announcement comes a day after Nato's supreme commander, General Wesley Clark visited Albania which is playing host to a growing force of US troops.

During his visit General Clark announced the imminent deployment of 24 Apache attack helicopters for use against Serb forces in Kosovo, although their arrival in Tirana has since been delayed by bad weather.

Yugoslav puts pressure on neighbours

The BBC's Jackie Rowland in Belgrade: "Unbearable strain caused by links between the KLA and Albania"

Yugoslav Foreign Minister Zivadin Jovanovic accused Albania of carrying out aggression against Yugoslav territory in Kosovo and said there was no purpose in maintaining relations.

He also said large quantities of military equipment were being smuggled into Kosovo.

At the same time the Yugoslav army is moving against rebellious figures in the government of Montenegro, the junior partner in the Yugoslav federation which has been trying to stay neutral in the conflict.

Fierce critic

The Yugoslav military prosecutor is seeking to arrest the Montenegrin deputy prime minister, Novak Kilibarda, a fierce critic of the Yugoslav president, Slobodan Milosevic.

Mr Kilibarda has been accused of undermining Yugoslavia's defence and military capabilities by encouraging army reservists to dodge the draft.

Officials of the Montenegrin parliamentary assembly say the military has no authority to strip Mr Kilibarda of his parliamentary immunity.

In Montenegro itself more refugees have continued to flood across the border from Kosovo, adding to the 63,000 that have arrived there since the bombing began. They have brought with them further allegations of Serb brutality.

One column of refugees arriving from the city of Mitrovica in northern Kosovo told reporters they had travelled for more than 24 hours after being forced from their homes by Serb security forces.

'Leave now'

US Apache helicopters are due to be deployed soon

Most had to leave their possessions behind and said the Serbs had shot at least two men for not obeying their orders to leave quickly enough.

One eyewitness saw the body of a decapitated woman lying by the roadside. Another said that a Serb policeman told him, "Don't come back, leave now. Ask Bill Clinton and Tony Blair to look after you."

Earlier as refugees continued to enter Albania five Kosovan refugees including three children were killed when their car hit a landmine in the no-man's land between the Serb and Albanian sides of the border.

No warning

The BBC's Kevin Connolly: "A major blow in the campaign"

Serb troops have been seen laying mines either side of the road in the past few days, but relatives of the dead say they were not warned of the dangers.

Serbian troops were also reported to have shelled a line of refugees, killing one and injuring 22 at the same border crossing at Morina.

The refinery at Novi Sad burns

More than 23,000 Kosovo Albanians passed into Albania on Saturday, taking the total to around 375,000.

Nearly three-quarters of a million people are estimated to have fled Kosovo since the crisis began in March last year.

• Kosovo 'grave gangs' used, says Nato
• Serbian toxic fumes fear
• Mine kills five refugees
• Refugee exodus continues