Jonathan Charles: Politicians fly in to deliver praise amid fading hopes for a short war

Mr Robertson told pilots at the Gioia del Colle air base that their work was vital if the murderus Serb offensive against Kovovo Albanians was to be stopped. He said their actions were "awe-inspiring", risking their lives every night for a cause.

The message to Yugoslave President Slobodan Milosevic from Mr Robertson was "Stop the violence now".

He accused the Serb troops of "outrages against humanity", and vowed that those responsible would be hunted down and put on trial.

"They will never be forgotten. They will be brought to justice one day if they continue." Reinforcements for the RAF Harriers, which successfully bombed Serbian targets on Sunday night, joined Operation Allied Force on Monday in anticipation of further raids.

Flying from RAF Wittering, Cambridgeshire, four Harriers landed at the Gioia del Colle base shortly after 1730 GMT on Sunday. More pilots arrived on board three Hercules transporter planes.

They join the eight Harriers already involved in the military mission, which aims to knock out Serb troops and paramilitaries directly engaged in repression in Kosovo.

Further attacks are expected on Monday following Nato's announcement that Serb tanks, armour, artillery and troops would be hit by air strikes if they were found south of the 44th parallel, 60 miles south of Belgrade.

Eight RAF Tornado ground attack planes, like those used in the Gulf before Christmas, were preparing to leave their base in Germany to join the aircraft at the southern Italy air base.

Nato's offensive is expanding to include mobile targets, including dangerous low-level attacks on Serb units concealed in difficult terrain.

Nato said the thousands of ethnic Albanian refugees crossing Kosovo's borders threatened a "humanitarian disaster" not seen in Europe since the end of the Second World War.

It said more than 500,000 refugees were now displaced in the worst population shift in Europe since 1945.

The RAF Harrier pilots who took part in Sunday's overnight raids were being debriefed at the air base. Senior officers said they had suffered no losses, but refused to give details.

Overnight raid

Six Harriers attacked the Serb targets, while two joined them as "spares". More than 60 Nato planes, largely F15 and F16 fighters, flew on the missions from Italian bases.

Two bombs struck a military airport outside Podgorica in Montenegro and a third hit a target close to the nearby Danilovgrad military warehouse.

A military airport at Nis in south-east Serbia and two airports near Yugoslavia's capital Belgrade were hit during the fifth wave of Nato air raids.

With further, more dangerous raids expected, Group Captain Ian Travers Smith spoke of the rigorous training needed for low-level flying.

Silhouettes of weapons, aircraft and ships used in the Kosovo crisis

"Setting aside the threat, low-level flying is dangerous at the best of times. That is why we have to train so hard and so long in that skill so we do it intuitively," he said.

Pilots on call at the base admitted they were nervous after the shooting down of an American Stealth Bomber on Saturday. But they said they felt reassured after the pilot was rescued by US troops.

Protest in London

Police in riot gear cordoned off central London as protesters supporting Albania stood just yards away from pro-Serb demonstrators.

A police spokesman said about 1,000 pro-Albanian demonstators gathered in Trafalgar Square, waving British, US and Kosovo Liberation Army flags.

They chanted "Free, free Kosovo" in support of Nato's air strikes against Serb targets.