"The outcome of this campaign is not in doubt, it's only a matter of how long it takes Nato to reverse Milosevic's campaign of ethnic cleansing," he told a Ministry of Defence briefing on Monday morning.

"We never said that this operation would be quick, but I can assure you it will be decisive.

"Nato has the will, the resources and the ability to continue this campaign for as long as it takes."

Mr Robertson said both Albanian government ministers and the United Nations Secretary General Kofi Annan had recently given their backing to Nato's demands on President Milosevic.

"With the international community united and increasingly vocal, it's only a matter of time for Milosevic."

The increase in allied forces on hand for attacks on Serbia showed the determination by Nato, the defence secretary said.

"Alliance foreign ministers are united in their determination that Milosevic must not be allowed to profit from his brutal policy.

"Round the clock air patrols will mean that Milosevic's thugs will be under fire from Nato aircraft both day and night."

'Cracks begining to show'

RAF Harriers are now able to drop bombs on targets even during heavy cloud, General Sir Charles Guthrie told the briefing.

A Nato photo showing suspected mass graves

The defence secretary also criticised Serb propaganda, such as reports of 15,000 soldiers having deserted to Greece and Bulgaria.

"I think everybody knows who's telling the truth and who isn't," Mr Robertson said.

He earlier insisted President Milosevic was coming close to admitting defeat.

"What I do believe is that Milosevic is wakening, that there are cracks in Belgrade," he said.

"There are clear indications we are getting, not just through these offers that are now being made each week to sue for peace on his terms, but also indications that we are receiving ourselves that his military commanders believe this is a suicide policy that will be a stain on the Serbian nation forever."

Blair defends bombing

The UK prime minister earlier made a new defence of Nato's air campaign against Yugoslavia.

The BBC's John Devitt says the PM writes there is no alternative to taking action

In an article in the international magazine, Newsweek, Tony Blair says President Slobodan Milosevic's actions have given rise to suffering people thought had been banished from Europe forever.

Mr Blair said Europe and America "must stand firm together" to defeat and reverse ethnic cleansing in Kosovo.

Doing nothing in response to the alleged atrocities perpetrated by Serbian President Slobodan Milosevic "would have been to acquiesce in brutality", he said.

"We have learnt by bitter experience not to appease dictators," the prime minister continued.

"We tried it 60 years ago. It didn't work then and it shouldn't be tried now.

Foreign ministers consider strategy

Mr Blair's comments came as Nato's 19 foreign ministers gather in Brussels to discuss progress in the war with Serbia and reaffirm their intention to continue.

The Nato meeting will be the first of a series of meetings this week seeking to find a way forward in the Kosovo crisis.

On Tuesday US Secretary of State Madeleine Albright is due to meet Russian Foreign Minister Igor Ivanov in Oslo in an effort to avoid a further deterioration in relations on the Kosovo issue.

Later in the week, European Union heads of government are also expected to meet to discuss Kosovo.