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UK Defence Secretary George Robertson has ruled out any suggestion of using Nato ground troops to stop alleged "ethnic cleansing" in Kosovo.

His comments come as the Serbs show television pictures of three US soldiers captured by them near the Macedonian border on Wednesday.

Mr Robertson told BBC Radio 4's Today programme: "We have obviously got to assess carefully what's happened, but it is an ominous development that has taken place.

The BBC's Alan Grady : The soldiers were not involved in Operation Allied Force

"It is all the more ominous because these troops were there as part of the former UN peace-keeping force on the border between Macedonia and Kosovo.

"They were there with no hostile intent at all. They were under a UN mandate until the beginning of last month.

George Robertson in debate with the BBC's John Humphrys on the deployment of ground troops

"They were on the wind-down and going home, having performed that function for some time," he said.

Mr Robertson said: "If the Serbs are now in the business of taking on people who are there for peace-keeping purposes, then they are sinking again to low depths."

He said the 19 countries involved in Operation Allied Force planned to continue with the "long haul" strategy of air strikes despite the comments of some "armchair critics".

Mr Robertson said: "By military force against military bases and against the military machine, we will achieve our objectives.

"It can't be done overnight. There is no simple, easy knock-out blow that can be done."

He said the idea of putting ground troops into Kosovo to restrain Serb forces carrying out "ethnic cleansing" appeared attractive.

But he said several factors made the idea a non-starter:

• It would involve a great deal of preparation time, allowing Mr Milosevic to "finish the job".

• There was a danger of "colossal casualties" among Nato troops, Serbian forces and Kosovar Albanian citizens.

• There was a threat of creating a wider conflict.

He said: "A lot of careful thought went into the strategy that we have adopted and we are confident that that is the best method of dealing with it.

A US B-1 bomber takes off from South Dakota on its way to RAF Fairford

"It's going to be long, it's going to be difficult. We've got the stomach and the determination to do it and we believe that it will be successful," he added.

Latest figures suggest that Kosovo Albanian refugees continue to flood across the borders, with an estimated 90,000 in Albania, 20,000 in Montenegro and 15,000 in Macedonia.

George Robertson: "Belgrade is getting the message"

The UK is spending £10m on relief for refugees. On Thursday, the latest of several UK aid flights is to leave Copenhagen for the Albanian capital, Tirana.

Flights will leave from different European and African locations where crisis supplies have been assembled.