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Nato has launched a fourth day of air strikes against Yugoslavia amid extreme concern among members of the alliance at reports of growing numbers of Serb atrocities against Albanians in Kosovo.

A senior Nato official in Washington said the strikes began on Saturday with a Tomahawk cruise missile launched onto the vicinity of the capital, Belgrade.

US officials later reported that American warships in the Adriatic had launched two Tomahawk missiles in daylight.

John Simpson reports from Belgrade: "No pictorial evidence of any massacres"

Air-raid sirens sounded in Belgrade and in Pristina and large explosions were reported in the Yugoslav capital. Witnesses reported scores of people rushing to bomb shelters after the sirens sounded and the explosions began.

'Stay the course'

Nato members said reports of Serb atrocities in Kosovo had only strengthened their resolve to continue the air strikes on Yugoslavia.

US President Bill Clinton on Saturday urged Nato forces to "stay the course" and carry on attacking Serb targets.

Since the raids began on Wednesday, "Serb troops have continued attacks on unarmed men, women and children," President Clinton said.

"That is all the more reason for us to stay the course," he said in his weekly radio address.

His comments were echoed by UK Prime Minister Tony Blair, who said Nato air strikes were "the only chance of securing an end to this brutality" in Kosovo.

UK Prime Minister Tony Blair: "Air strikes have to continue until we reach our objective"

He said: "There's no doubt at all in my mind that having committed ourselves to this action we have to see it through and we have to continue with the air strikes we have launched until Milosevic is brought into line with the proper standards of civilised conduct."

The prime minister also warned President Milosevic that if he attacked Nato forces elsewhere in the Balkans, "the retaliation will be swift and very, very severe".

Nato 'despotism'

The Yugoslav leader appeared unbowed as a fourth day of air raids got under way. He said Nato "despotism" must be resisted.

Speaking on state television, President Milosevic said the Nato raids were ruining the United Nations system, and were the worst threat to peace since the Second World War ended in 1945.

"The forces of peace (will prevail) ... over the forces that want to organise the world by the dictates of force and crimes against nations that do not accept being subjugated," he said.

But on Saturday, Nato was itself declaring that reports from a variety of sources indicated that "dark things are happening" in Kosovo.

Spokesman Jamie Shea said there was no firm evidence yet, but reports talked of intimidation and harassment and widespread violence against the Kosovan population. "These are extremely worrying indications," he said.

Nato's Jamie Shea tells of reports of Serb atrocities

During a third night of raids, around 40% of Nato targets were inside Kosovo itself, according to the Pentagon - double the percentage in the previous two days' strikes.

At least half-a-dozen explosions were reported late on Friday in Kosovo's capital Pristina, and raids were also reported in other towns in the province.

Nato forces also inflicted a sustained overnight attack on the Yugoslav capital, Belgrade.

'Appalling violence'

At a briefing about the third wave of strikes, UK Defence Secretary George Robertson said that Yugoslav President Slobodan Milosevic was continuing with his "brutal campaign of repression".

Protests in London on Saturday against Nato attacks

Mr Robertson said: "The UN High Commission for Refugees - with all its considerable authority - was told by refugees yesterday of appalling violence committed against the Kosova people.

"The Serbs are bombarding villages to the point of obliteration. We have heard that some villages do not exist," he said.

The defence secretary told of consistent reports of extra-judicial killings by Serb security forces and added that 30,000 people were reported to be out in the open around Podujevo, Kosovska, Mitrovica and Shale, with thousands more heading for the border.

Other unconfirmed reports of mass killings by Serb forces have been emerging and Kosovo Liberation Army (KLA) sources were reported as saying on Saturday that Serb paramilitary gangs had slaughtered several hundred Kosovo Albanians in Djakovica overnight.

Serbian officials said the KLA had stepped up its attacks on Serb forces in Kosovo.

There was no independent means of checking the reports. Most foreign journalists were ordered out of the country on Thursday, and international monitors left the province before the bombings began.