His statement came as US President Bill Clinton approved the call up of 33,102 reservists for active duty in the campaign.

Nato leaders are sticking to their policy that the air offensive against Yugoslavia will be enough to force President Milosevic to back down but have asked commanders to update contingency plans for a land invasion.

The president said he expects Defence Secretary William Cohen to call up reservists from all branches of the military to "fill critical support positions in our humanitarian and combat operations" in and around Yugoslavia.

Around 2,000 members of the Air National Guard and Air Force Reserve are expected to be the first to be mobilised.

'Noose tightens'

The BBC's Brian Hanrahan: "Diplomacy has been resumed at a leisurely pace"

Meanwhile a force of about 2,000 British soldiers supported by tanks and artillery guns have arrived in the Greek port city of Thessaloniki to join other Nato troops in in the neighbouring former Yugoslav republic of Macedonia.

"The noose around Yugoslavia is tightening," General Clark told a press briefing at Nato headquarters on Tuesday.

General Clark say Yugoslav defections are increasing

He said the campaign so far had been very destructive to the infrastructure supporting President Milosevic.

"This is a campaign that's working," General Clark told reporters adding that the air campaign would be intensified in line with an agreement by alliance leaders at the weekend.

The BBC's Mark Laity: "The Summit has reinforced the belief in air power"

The Yugoslav authorities say more than $10bn worth of damage has been caused, and about half-a-million people have lost their jobs as a result.

The BBC's correspondent in Belgrade, Jackie Rowland, says some economists estimate that the financial damage is greater than the losses sustained by Yugoslavia during the whole of World War II.

Oil embargo

Yugoslav officials say billions of dollars of damage have been done

Meanwhile Nato officials have been discussing ways of further depleting the Yugoslav military by blocking oil shipments to the country.

Nato says the number of ships unloading oil into Yugoslav ports has increased significantly since the idea of an oil embargo was first proposed.

General Clark said up to 10 tankers a day were docking at the Montenegrin port of Bar, though this has been hotly denied by the port authorities.

France is among a number alliance members to have expressed reservations about the plan saying that it would be illegal without a specific UN resolution and could spark a confrontation with Russia.

On Tuesday General Clark told reporters he believed that any oil embargo should be backed up with the threat of force.

"Any visit-and-search regime of course has to have appropriate rules of engagement," he said.

BBC Moscow Correspondent Andrew Harding: "Russia is being groomed as a potential peace broker"

Earlier Russian Foreign Minister Igor Ivanov warned that a Nato-led oil blockade would enflame the crisis.

Mr Ivanov was speaking after talks in Moscow with the US Deputy Secretary of State Strobe Talbott, who described the talks as open, serious and constructive, but gave few details.

Civilian area 'hit'

In the latest Nato raids, Serbian officials say at least 16 civilians were killed in an attack on a residential area in the southern town of Surdulica.

Yugoslav officials say a number of homes were destroyed when two missiles hit the centre of town.

Nato said it had struck an army barracks, but declined to rule out it might have killed civilians in the strike.

As yet there has been no independent confirmation of these reports, correspondents say the authorities are organising a convoy of vehicles to take journalists to the scene.

Meanwhile, aid agencies say the lives of ethnic Albanian refugees sheltering in camps along the Kosovo Albanian border are in danger because of increased Serb military activity.

Serbian snipers and artillery have been targeting Kosovo Liberation Army positions close to some of the camps, where 50,000-80,000 refugees have arrived over the past few weeks.

Other top stories

• Red Cross sees US soldiers
• Cook: Serb morale collapsing
• Refugees tell of three-day rape ordeal
• Talks bid to stop Kosovo suffering
• Analysis: Cracks in Belgrade leadership