Defence Secretary George Robertson told the daily Ministry of Defence briefing that the number of refugees arriving in neighbouring countries had greatly increased, after a period of relative calm.

More than 15,000 ethnic Albanians left Kosovo over the weekend, he said.

War crimes co-ordinator David Gowan will seek evidence of atrocities by Serb forces to hand to the International War Crimes Tribunal.

Mr Robertson said Britain would do its utmost to bring the perpetrators of such crimes before a court.

"We pledge to them [the refugees] to do everything in our power to ensure the people who tortured and mistreated them face justice," he said.

A Downing Street spokesman said the decision to send Mr Gowan to Albania stemmed from Prime Minister Tony Blair's experiences during his two visits to refugee camps there.

George Robertson: Promising justice to refugees

"The prime minister was horrified by the accounts of how they [the refugees] had been treated by the Serb authorities.

"He has asked David Gowan to go to Albania to interview these latest refugees to help gather information for the tribunal.

"We will do all we can to bring to justice those engaged in torture and ill-treatment."

Mr Gowan has already been gathering evidence from refugees who have arrived in Britain.

Chief of the Defence Staff General Sir Charles Guthrie said 430,000 refugees remained in Albania and 250,000 in Macedonia, despite efforts to move them out of these countries.

Kosovo will 'strain' army

UK Defence Secretary George Robertson earlier said that operations in the Balkans will place the British Army under "substantial strain".

Mr Robertson's remarks come in the same week as Nato commanders discuss the numbers of troops needed to escort the refugees back to Kosovo.

The defence secretary told the BBC: "There will be substantial strain, but at the same time the army is there for operations.

"We will have to cope with the strain, and so will all the people and their families. I have no doubt that we will be able do so."

As the campaign in the Balkans continues original estimates that around 20,000 troops will be needed to ensure the safe return of the Kosovan refugees are being revised upwards.

This is partly due to the damage inflicted on Kosovo by the Serbs as well as the damage to the province's infrastructure caused by Nato's air strikes.

Mr Robertson said that when the time came to send in troops, if Nato was "looking at something slightly less ordered than a permissive environment, more troops will be needed".

Defence Secretary George Robertson: "Lightly armed troops are not the answer to the problems of the Balkans"

He elaborated, saying ground forces may be sent into a "situation where the Serb troops may well have stopped fighting but that people's lives might still be at risk".

He said it was "wise to build into any planning some worst cases".

Speaking over the weekend, UK Foreign Secretary Robin Cook said the US Secretary of State Madeleine Albright had endorsed the idea of deploying ground troops in Kosovo in a "non-permissive" environment.