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Tens of thousands of refugees fleeing Kosovo are describing horrifying stories of Serb atrocities.

In a desperate race to halt the Serb military machine, Nato officials say around the clock attacks are being mounted and have confirmed it has been using special A-10 "tankbuster" planes.

Nato says the humanitarian crisis is deepening and a number of leading Kosovo Albanians have been executed.

Clarence Mitchell reports on the refugee influx into Northern Albania

Belgrade says its forces are acting only against the rebel Kosovo Liberation Army and blames Nato for what is happening in the province.

According to Nato, Yugoslav forces are attempting to ethnically re-engineer Kosovo's political make-up as they force refugees out of Kosovo.

Refugees arriving in Macedonia, Albania and the Yugoslav Republic of Montenegro say:

• Security forces gave them minutes to leave their homes or be shot.
• Whole families were murdered with men of fighting age a particular target.
• Bodies are lining the routes to the borders.

A Kosovar Albanian refugee in Kukes, Macedonia explains why he left

Our correspondent on the Kosovo-Macedonia border, Paul Wood, says the stories of atrocities are consistent and credible.

Reliable monitors report that refugees are crossing the border into Albania at the rate of 4,000 people per hour.

Offensive steps up

The Nato offensive is stepping up a gear despite bad weather. Nato sources say the air strikes have been going on all day - the sixth day of strikes - with much more intensity than before.

Paul Welch reports from Montenegro: The stream is becoming an exodus

BBC Defence Correspondent Mark Laity says Nato is now operating on a 24-hour cycle, with the main focus centred on Kosovo.

The Pentagon says it is beefing up its air power over Yugoslavia, deploying five long-range B-1 bombers and five more EA-6B Prowler aircraft.

The sixth night of bombings got underway with Nato planes taking off from Aviano airbase in Italy and explosions heard just north of Belgrade.

Witnesses said detonations occurred in the direction of Pancevo near military installations shortly after air raid warnings sounded at 1848 GMT.

Serb news agencies are reporting three powerful explosions near Podgorica, the capital of neighbouring Montenegro. Serb radio says a Nato plane - believed to be a Harrier - was downed over Podgorica early on Tuesday morning. There is no independent confirmation of this.

Russian mission

In Moscow, Russian President Yeltsin said he would send his prime minister on a peace mission to Belgrade. Yevgeny Primakov is expected on Tuesday.

The BBC Moscow Correspondent Andrew Harding says it is not clear what new proposals Moscow can now put forward or whether it intends to apply real pressure on President Milosevic or simply show solidarity with him.

The European Union's outgoing commissioner for humanitarian aid, Emma Bonino, said she would also be visiting the region later this week.

Yugoslvia's deputy Prime Minister Vuk Draskovic: This present tragedy was prompted by Nato

French and British leaders have made strongly-worded attacks on the Yugoslav president for his role in the conflict in Kosovo.

The British Prime Minister, Tony Blair, said Mr Milosevic should be made to pay a high price for "every act of barbarity and slaughter of the innocents".

French President Jacques Chirac said Mr Milosevic is responsible for more than 200,000 deaths and millions of displaced people during conflicts in the Balkans over the past 10 years.

"Today we must stop the spiral of barbarity and take away from this regime the means to conduct such operations," Mr Chirac told the French people in a televised address.

Assassination reports

Nato spokesman Jamie Shea said Belgrade was seeking to destabilise the entire region by sending a flood of refugees into neighbouring countries.

Nato says Serbia is attempting to cancel out the identity of the refugees by withdrawing their passports and car number plates.

Diplomatic Correspondent Bridget Kendall: Serb TV is not reporting the refugee crisis

Air-Commodore David Wilby, citing "reliable sources", said Serbs had killed Fehmi Agani, the main adviser of the Kosovo Albanian leader, Ibrahim Rugova.

Mr Agani led the Kosovo Albanian delegation at the peace talks in Paris.

Air-Commodore Wilby said the editor-in-chief of the Kosovar daily newspaper Koha Ditore, Veton Surroi, was among those "executed".

Plea for help

Albania and Macedonia, which have been receiving tens of thousands of refugees have appealed for urgent international help to provide for the refugees.

The Organisation of Security and Cooperation in Europe (OSCE) announced that a total of more than 60,000 refugees had arrived at the border with Albania in the space of 36 hours.

People are continuing to arrive at the Albanian border in their thousands. Refugees are also streaming out of Kosovo into Montenegro, in the north.

A spokesperson for the UN High Commissioner for Refugees, Lyndall Sachs, said giving definite figures was difficult because there were no longer monitors on the ground.

Zakaria Kawi of UNHCR explains how aid agencies are dealing with refugees

But the UNHCR believes that up to 25% of the population of Kosovo - more than 500,000 people - have fled their homes since the conflict began.

"This is the figure which has been put together over the past year since this conflict broke out. So a lot of those were displaced before the Nato offensive," Lyndall Sachs said.

Serbia denies crisis

A refugee tragedy unfolding

But the Serbian Government refugee commissioner, Bratislava Morina, who is of Albanian origin herself, has dismissed reports of a refugee crisis in Kosovo.

"There is no humanitarian catastrophe in Kosovo whatsoever," she said in a television interview.

There is still no sign that Serbian President Slobodan Milosevic is prepared to back down in the face of Nato's air campaign.