The overnight attack followed reports on Tuesday of Yugoslav troop action on the Albanian and Croatia borders and in Montenegro - Serbia's junior partner in federal Yugoslavia.

The 20-storey building - which also houses a number of local radio and television stations - was severely damaged as three massive explosions shook the city.

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Witnesses reported that at least 15 people had been in the building when the attack took place at around 0300 on Wednesday.

A BBC correspondent, at the scene, said the building's top and many of its lower floors were in flames.

Serb TV showed the bridge under attack

Yugoslav media also reported a series of attacks on northern and central Serbia, including the last bridge over the Danube in the town of Novi Sad. Serb television said two Nato missiles hit the bridge.

A military and civilian airport in Ponikve in central Serbia also came under sustained attack.

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The latest strikes came after a day in which Yugoslav and Albanian troops exchanged cross-border fire for several hours. It was the first clash between the two armies since the start of the Kosovo crisis nearly four weeks ago.

An OSCE monitor said the skirmish had been caused by Serb provocation. One Albanian soldier was reported injured.

The Kosovo Liberation Army is known to be active along the border trying to infiltrate its fighters.

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Tension has also increased between Serbia and Montenegro, with reports that Serb forces have closed Montenegro's border with Croatia and demanded control over the Montenegrin police.

Police at local border crossings had been allowing Westerners, including journalists, into Montenegro without Yugoslav visas. The army has seen this as part of a pattern of disloyalty from Montenegro.

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

Elsewhere, Croatia complained to the UN that at least 200 Yugoslav soldiers had crossed into a disputed strip of land on its border with Montenegro.

The Prevlaka peninsular controls access to an important Yugoslav naval base in Montenegro. UN monitors withdrew when the Nato bombing began.

A BBC correspondent in the region said it was thought that the Serbs might be trying to encircle Kosovo guerrillas.

Earlier, Nato officials said Yugoslav security forces attacked three ethnic Albanian villages inside Montenegro, close to the border with Kosovo. Six villagers were reported killed.

Macedonia border reopens

The Macedonian authorities, meanwhile, have allowed the United Nations to start transporting some 7,000 ethnic Albanian refugees from the country's border zone with Kosovo.

The refugees had been stranded in a no-man's land between the frontiers without food and blankets since Monday night.

Macedonia had refused them entry, saying that the existing reception camps are already overcrowded.

However the border was opened after talks between the UN refugee agency and Macedonian officials.

(Click here for a map showing latest strikes)

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Officials in Yugoslavia say the damage from Nato's bombing campaign has cost around $100bn dollars so far.