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Cruise missiles have pounded the Yugoslav capital Belgrade for the third consecutive night, as the United States announces it is sending 24 attack helicopters to bolster allied forces.

Serb media reports said the headquarters of the Yugoslav Air Force and anti-aircraft command at Zemun in the northwest suburbs of the capital Belgrade had been hit.

Eyewitnesses aboard American warships in the Adriatic said some 20 Tomahawk missiles were launched towards Yugoslavia.

The Serb media says aircraft have also bombed an airfield southwest of the Kosovo capital, Pristina, and an army barracks in the town of Raska, about 100 miles south of Belgrade.

Apaches can be used for night attacks

Other reported targets included a bridge on the Ibar river and both ends of a railway tunnel in the Ibar valley. Blasts have also been heard in Novi Sad in northern Serbia and the eastern Kosovo town of Gnjilane.

Nato spokesman Jamie Shea said it was "a very successful and extensive night of operations."

In an interview with BBC World, he said a change in the weather enabled Nato to step up its activities against infrastructure targets, and against Yugoslav ground deployments in Kosovo itself.

Jamie Shea: "We had a successful night of operation"

Earlier, the US announced it was boosting air operations with the deployment of 24 Apache helicopters and 2,000 extra troops.

The helicopters, which can operate in all weathers, are low-flying attack aircraft used to fight ground troops and tanks. They are expected to be deployed against forces shelling Kosovo villages.

Stephen Gibbs: "The alliance is targeting Yugoslav infrastructure"

US Pentagon spokesman Ken Bacon described the decision as a logical expansion of the current Nato air campaign and denied that it was a step towards deploying ground forces.

Refugees offered sanctuary

Food supplies are scarce

Meanwhile, hundreds of refugees are beginning to pass through a relief camp in Macedonia, close to the border with Kosovo. The camp was set up by Nato, after the former Yugoslav republic said it could not take any more refugees.

United Nations relief workers will provide medical care for up to 100,000 refugees, and arrange transport to other countries until they can return to Kosovo.

Germany, Turkey, Britain and the United States are among those who have offered sanctuary.

Jon Leyne: "Nato has been forced into a dramatic change of policy"

But the Italian Prime Minister, Massimo D'Alema, on a visit to Albania, said dispersing them through Europe would make a success of ethnic cleansing.

And he praised the government there for letting all its refugees stay.

His fears were echoed by European Union Humanitarian Aid Commissioner Emma Bonino. "We should not disperse people all over. We should not co-operate in any way with ethnic cleansing," she added.

The Pentagon's Ken Bacon: "This is a pure and simple expansion of the air operation"

EU officials are to hold an emergency meetings in Brussels to co-ordinate a plan to provide temporary shelter to the Kosovo refugees.

German Chancellor Gerhard Schröder stressed there should be a "fair division of the burden" to avoid the mistakes made during the Bosnian conflict.

Germany has offered to take 40,000 Kosovo Albanians, and the United States and Turkey 20,000 each. The UK says it will take several thousand refugees.

But Nato says the long-term objective is for the refugees to return home.

At least 360,000 Kosovo Albanians have poured out of Kosovo since the air strikes began. Nato says at this rate the Serbs will empty the province within 19 days.

The UN refugee agency estimates 204,000 people have fled or been expelled to Albania and 115,000 to Macedonia since 24 March. Disease is spreading and refugees are dying at both borders.

'Serb war machine damaged'

Assessing operations so far, Nato says its bombing raids are cutting deep into the Yugoslav war machine, causing shortages of fuel and other vital supplies.

Nato says the latest reports from Kosovo suggest Yugoslav forces are moving towards the Albanian border to engage the rebel Kosovo Liberation Army.

KLA spokesman, 'Drini': "Guerrilla warfare is the best way we can fight"

A KLA spokesman has said "successful operations" are being mounted against Serb forces.

Nato said strikes on Belgrade on Saturday night targeted an army headquarters, an oil storage centre, a police academy and a heating plant.

Outside the capital, bridges and an ammunition plant were also hit. The Yugoslavs said three people were killed.

Coup fears

A child gives the Serb salute

Meanwhile, in Montenegro, speakers at an open air pro-Serbian concert denounced Nato and called for stronger unity between the two republics in federal Yugoslavia.

Officials feared the concert could be hijacked by Serb nationalists plotting a coup.

In another development, a senior Serb official has said the three US soldiers captured by Serb forces will not go on trial and will be ''sent home once this stupidity ends''. But the Yugoslav ambassador to the UN has said a trial is still possible.