Speaking on the BBC's Today programme Mr Cook said: "We are now widening that range of military targets ... they will be directly relevant to the military forces in Kosovo."

However, bad weather has again prevented British Harriers from carrying out bombing raids on Serbian targets in Kosovo.

The RAF said a number of Harrier GR7s took off from the Gioia del Colle base, in Southern Italy, but thick cloud thwarted them for the second night running.

The attacks are continuing after Nato rejected an offer from the Serbs to withdraw some troops from Kosovo.

The BBC's Jonathan Charles reports on the latest RAF set-back

But in a week of bombing, the RAF Harriers have completed only two successful missions.

Our correspondent in the area, Jonathan Charles, says the weather is proving deeply frustrating for the RAF.

Group Captain Ian Travers Smith said the Harriers launched at about midnight local time.

Exodus: Harriers aiming for those attacking refugees

"This time though they made it all the way to the target area to find cloud cover over the target," he said.

"Dealing with weather as an aviator is a constant thing and even in peacetime when you are trying to achieve a certain amount of training it can be annoying."

He added: "The circumstances at the moment is a weather system that seems to be sucking moist weather into the area."

The planes also came under some of the heaviest anti-aircraft fire yet.

The mission was the second strike mounted on Tuesday.

RAF Harriers earlier flew their first daylight raids since air strikes against Serbian targets began but no details have been given.

Serb offer rejected

Earlier the UK Government joined its Nato allies in rejecting Yugoslav President Slobodan Milosevic's conditional proposal for an end to the conflict.

Preparations: 24 hour attacks

Speaking after efforts by Russian leaders to secure an end to the bombing produced movement from the Serbs, Foreign Secretary Robin Cook said that Mr Milosevic's proposals fell far short of what Nato wanted to see.

In a statement issued by Yugoslav state television, Mr Milosevic said he was ready to withdraw some forces from Kosovo if Nato halts air strikes.

Defence Secretary George Robertson called the proposal neither "a sensible nor a serious offer".

Defence Secretary George Robertson: "This offer is not serious"

"He is not saying he'll stop the killing," he told BBC News. "He is not saying he will bring to troops back to where they were last October.

"I think it is indication that what is involved here is a bit of game - Milosevic making what looks like an offer on the face of it but which does not involve a cease-fire."

Pressure over aid

The UK is leading international efforts to aid refugees and an RAF plane carrying 42 tonnes of blankets and other supplies left for the region on Tuesday.

But putting fresh pressure on the cross-party support for the Nato action, Conservative leader William Hague accused the Government of doing "too little too late" to help the refugees.

In a statement, Mr Hague said: "Whole villages have been uprooted, families with young children find themselves thrown out on to exposed roads, and elderly people are facing night after night out in the cold.

"Nato must have foreseen this kind of reaction by Milosevic to their present military action and must have made contingency plans.

"However, I find it disturbing to read reports that the government and its allies have made little or no preparations to deal with the human tidal wave now on the move in the Balkans.