The BBC's Jonathan Charles in southern Italy: "The more the pilots fly the more they see this is not a risk-free operation"

Eight Harrier jump jets joined in the latest raids and all returned safely to their base in southern Italy.

About 60 Nato planes flew sorties over Yugoslavia, hitting tanks and troop transporters as well as fixed targets such as barracks and police stations.

Belgrade was largely spared, with the main focus of the Nato attack centred on targets in and around the Kosovan capital Pristina.

Nato says Serb tanks, armour, artillery and troops will be targeted if they are found south of the 44th parallel, which runs 60 miles south of Belgrade.

French Defence Minister Alain Richard said the aim of the raids was to "attack the skeleton of Serbia's military and repressive machine".

Albanian exodus

The intensification of the air war comes amid claims that Serbian war crimes in Kosovo are creating Europe's worst refugee crisis since World War II.

Thousands of Kosovars are streaming across the border into Macedonia and Albania and giving consistent accounts of atrocities by Serbian forces.

Yugoslav government sources say only "one in 1,000" stories of atrocities is true and claim many of those leaving Kosovo are fleeing the bombing.

As Operation Allied Force enters its sixth day, Western political leaders said they would continue the air raids.

Serbian TV shows a police station in Pristina on fire

Twelve more British combat aircrafts are being sent to southern Italy to join the offensive. Four Harriers were sent out on Sunday night with eight Tornado GR1 bombers set to join them this week.

Protected by cloud cover

Group Captain Ian Travers Smith said the eight Harriers - only six of which carried bombs - may have been protected from anti-aircraft fire by cloudy weather in the Balkans.

He said: "The operation was successful and in my opinion the boys' tails are up."

But Gp Capt Travers Smith said the night's raid were "not without incident".

"We have seen incidents where we have been unlucky with the weather, but this time it looks like the weather may have helped us," he said.

A technician reads a paper in the cockpit of a Harrier in southern Italy

Morale among the RAF pilots is said to be high, partly as a result of the audacious rescue of the US airman whose Stealth bomber crashed inside "enemy" territory on Saturday.

'Morale booster'

One pilot, who declined to be named but had experience of operations in Bosnia and Iraq, said: "We are hugely relieved. It is such a morale-boosting thing."

Most of the Nato planes flew from the Aviano airbase in northern Italy but several US B-52 bombers took off from RAF Fairford in Gloucestershire.

UK Defence Secretary George Robertson says there is evidence the Serb warlord Arkan, who has been accused of war crimes in Bosnia, was moving his supporters into Kosovo.

Meanwhile Albania's President Rexhep Meidani has rallied to the defence of his ethnic brethren and called for Nato ground troops to be sent into Kosovo.

But Nato leaders appear to have little appetite for such an idea.