Alliance spokesman Jamie Shea said it was "new and distressing'' to hear of ethnic cleansing by Serbia inside Montenegro - its junior partner in federal Yugoslavia.

Montenegro has increasingly sought to distance itself from the policies of President Milosevic.

At the daily Nato briefing, Mr Shea said villages close to the border with Kosovo had been attacked in recent days.

''This can only exacerabate the problem, which is already of alarming proportions,'' he said.

Milisav Paic: "An attempt to demonise Yugoslavia"

Yugoslav Foreign Ministry adviser Milisav Paic told the BBC the Nato statement was an attempt "to fabricate events in order to demonise Yugoslavia".

Meanwhile, Croatia has complained to the UN that at least 200 Yugoslav soldiers have crossed from Montenegro into Croatian territory.

The Croatian ambassador to the UN, Ivan Simunovic, called on the Yugoslav forces to withdraw immediately.

The Prevlaka area on the Croatian side of the frontier, which is disputed by Yugoslavia, is a demilitarised zone monitored by the UN. The area commands the entry to Kotor Bay, where an important Yugoslav naval base is located.

UN monitors withdrew when the Nato bombing began.

Serb forces and Albanian troops are also reported to have exchanged fire on the Yugoslav-Albanian border in the first clash between the two armies since the start of the crisis.

'Children held for blood'

Mr Shea said refugees reported that Serb forces were:

• Holding 700 Kosovo-Albanian boys prisoner, using them as "blood banks" for injured troops.

• Using Kosovo Albanians as human shields, forcing them to stand in front of Serb tanks for two days.

He said upwards of 100,000 Albanian men of fighting age were unaccounted for in Kosovo. Reports from refugees suggested more than 3,500 had been executed, he added.

Mr Shea said the Serbs were mounting a ''safari operation'' against Kosovo-Albanians.

He said the Serbs were:

• Shelling into hills, forcing refugees into the open and beating them out of bushes.

• Clogging up roads, moving refugees around and mixing them with military vehicles.

• Putting them into trains and sending them to the border, sometimes turning them back again.

Mr Shea said Serb strategy appeared to be to drive people to the south of Kosovo, herding them near to the border, but not allowing them to cross.

''It is as if Milosevic is trying to develop a surge operation,'' he said.

Bombing 'just'

Following a 27th night of bombing raids against Serbian targets, the UK Prime Minister, Tony Blair, described the conflict in the region as "just".

It is a just cause and it is a cause that we will succeed in winning," Mr Blair said.

He also said it would continue until Nato's demands were met in full.

His words echoed those of Nato Secretary-General Javier Solana, who spoke just a few minutes before Mr Blair took the stand. "We will stand firm to the very end," said Mr Solana.

(Click here for a map showing latest strikes)

Nicholas Witchell: "Little sign of the Serbs being deterred from ethnic cleansing"

In another development, the UK Foreign Secretary, Robin Cook, said the UK would allow the largest-ever release of intelligence information in order to asssist war crimes investigations.

The US announced that an extra 500 paratroopers are on their way to Albania.

US officals said the fresh forces would support Nato's Task Force Hawk, the deployment of 24 Apache ground-attack helicopters which are on their way to Albania.

Russia calls for peace

Mike Williams: Targets appear to be industrial zones of Nis

Yugoslav President Slobodan Milosevic said on Tuesday he believed that the Russian Orthodox Church and Russia itself would contribute to bringing about an end to what he called the aggression against Yugoslavia.

He was speaking after talks in Belgrade with the visiting Patriarch of Moscow and All-Russia, Aleksiy II.

The UN agency, the World Food Programme, says an estimated 800,000 people are on the move in Kosovo.

Although the flood of refugees crossing into neighbouring countries had turned into a trickle, 3,000 have appeared overnight at the border with Macedonia.

The UN has tried to get aid to them but the Macedonian authorities say they will allow only 400 into the country, arguing that the camps are at full capacity.

• Blair: We'll fight to the end
• 'Trapped in Kosovo'
• Apache: the all-weather attacker
• US volunteers join Kosovo rebels
• In the belly of a B-52