At a news conference on Sunday, he said the ground attack aircraft would leave RAF Wittering in Cambridgeshire that afternoon. They took off just after 1500 BST.

They are joining a force of eight Harrier GR7s already flying missions out of Gioia del Colle airbase in southern Italy.

George Robertson: "We are going to hit them hard"

A Ministry of Defence spokeman said that since operation Allied Force began on Wednesday, those Harriers have destroyed two targets but operations have been hampered by poor weather.

"The RAF is playing a fully active part. However, if there is any doubt at all in the mind of a pilot before he launches 1000lbs of high explosive - if the target is obscured by smoke or weather - he will not release it," the spokesman said.

Andy Tighe reports: "Nato prepares for the next wave of air strikes"

Mr Robertson went on to say that eight Tornado GR1s were also being prepared for action against Serb forces in Kosovo.

And an RAF Tristar will join the Nato air-refuelling tanker fleet in a support role for the warplanes carrying out bombing missions.

The defence secretary said the United States was also increasing its commitment with additional B52s arriving at RAF Fairford in Gloucestershire.

Pilots have not dropped bombs due to poor weather

Mr Robertson said that Nato had shifted its attacks from the Yugoslav air defence system to direct attacks on Serbia's "military thugs".

"We are ... confronting a regime bent on genocide," said Mr Robertson, referring to reports that Serbian security forces are carrying out massacres and forcibly displacing thousands of Kosova Albanians.

He also said that paramilitary leader Arkan - "a notorious and obnoxious thug" implicated in Bosnian atrocities - was known to be operating in Kosovo.