Live coverage l Map of overnight Nato strikes

Nato says clearer weather on the 13th night of its campaign against Yugoslavia has allowed it to carry out the most intensive air attacks so far.

Allied missiles and bombs were reported to have hit roads, bridges, fuel depots and military command centres in an apparent attempt to cut the Yugoslav army's lines of supply and transport.

A Nato official quoted by Reuters news agency said: "We conducted four waves of strikes . . . We haven't done that many before."

The targets included the main road from Belgrade to the Kosovo capital, Pristina, an army HQ in the eastern city of Nis and a military barracks near Prizren in Kosovo, according to the official.

More details are expected later on Tuesday.

The attacks came as the US promised that Nato's bombing campaign would continue "unceasing and unrelenting" until Yugoslav President Slobodan Milosevic reversed his action in the troubled province of Kosovo.

Some of those displaced by the turmoil in Kosovo have begun to be airlifted out of refugee camps to other countries which have promised to accept them on a temporary basis.

Brian Hanrahan reports on the damage done by the air raids so far

The first of those refugees were reported to be arriving in Turkey, which has promised respite for 20,000 people escaping Serbian violence.

'Civilians killed'

Serbian television said that several civilians had been killed in a raid on the southern mining town of Aleksinac and broadcast pictures of what it said were destroyed houses in a residential area in the centre of the town.

Claire Doole: "The UNHCR has called for international commitment for the safety of refugees"

Reports of the casualties could not be independently confirmed, but local reports put the death toll in Aleksinac at five, and police and army officials estimated that at least 30 others were injured in what they said was a series of three missile attacks. More bodies were believed to buried in the rubble.

Serbia's official RTS television station said that about 10 buildings had been destroyed in the blasts, as well as a centre for emergencies and a medical dispensary.

The UK Defence Secretary George Robertson told a Minstry of Defence briefing that the allied strategy was not indiscriminate.

However, he said he had heard reports of the civilian casualties.

"If this is true, then this is of course deeply regrettable, but as we have always said despite all our efforts such casualties will inevitably occur in a campaign of this size and complexity," he said.

Nato has not yet officially confirmed or denied the attack.

'Most brutal strike'

There were reports of civilian casualties in Aleksinac

Serbian TV showed footage of a spectacular fire after the bombing of what it said was an oil refinery in the northern city of Novi Sad.

Tanjug said huge flames and heavy clouds of smoke could be seen above the oil refinery in what it called the "most brutal and strongest strike" on the city in recent days.

The state news agency said that Nato warplanes struck in or near at least five Serbian towns overnight and that a railway bridge over the river Danube linking the border town of Bogojevo, north-west of Belgrade, to Erdut, in Croatia, had also been hit.

Paul Royall reports: Nato is intensifying its bombing campaign

State television also reported that missiles hit a fuel depot in the northern town of Sombor. A huge fireball rose up from the south-east of the town where the depot is located, the television said.

Five big explosions rocked Serbia's second city Nis according to Tanjug.

Kosovo hit

The province of Kosovo came under fire, with reports that several missiles blasted a barracks at Prizren, Kosovo's second largest city. Tanjug said it was the fourth attack there since the Nato launched its air campaign on 24 March.

President Milosevic: Vowed to rebuild targets of Nato strikes

Nato missiles also hit a communications relay station near Kosovo's capital Pristina, and hit targets on the main road from Kosovo to Serbia, Tanjug said.

Other reports spoke of bombs landing in Pancevo, north of Belgrade, and the destruction of a major television transmitter which cut reception to the whole province of Vojvodina.

Two Nato planes were reported by Serbian TV to have been shot down during their attack on the Novi Sad area on Monday night. But alliance spokesman Lieutenant Colonel Michael Kaemmerer denied the report, saying all planes had returned to their bases.

Bombing to continue - Clinton

The intensified bombing raid came after US President Bill Clinton warned Belgrade that there would be more strikes to come.

Bill Clinton: "Empty promises won't do the job"

He said it would no longer be enough for President Milosevic to merely stop the alleged violence against Kosovo Albanians.

"A Kosovo denied its freedom and devoid of its people is not acceptable," the US president said.

"Our plan is to persist until we prevail."

However Mr Milosevic on Monday appeared unbowed as Nato stepped up its attacks, and was reported to have vowed to rebuild facilities damaged by the air strikes.