Until now, investigators have had only satellite photographs and eyewitness accounts to go on.

Evidence of massacres is coming to light as Nato forces move through the province.

British paratroops have found what they believe is a mass grave containing about 100 bodies in the southern village of Kacanik. Another has been seen by journalists in the central village of Mali Ribar.

The UK plans to send a team of 15 forensic experts and the US has said it has flown an initial group of 25 agents from the Federal Bureau of Investigation.

UN Secretary-General Kofi Annan has appealed for other member nations to join the hunt for evidence of alleged massacres and other crimes.

Evidence gathered will be passed to the International War Crimes Tribunal in The Hague.

Forensic teams

Two forensic experts from London are on their way to Kosovo to set up a headquarters for the investigations.

UK Home Secretary Jack Straw said: "Their forensic skills will be used to prepare evidence for the War Crimes Tribunal to bring the perpetrators of these vile crimes to justice."

And David Gowan, war crimes co-ordinator for the UK Foreign said: "The scale of the criminality is enormous. The number of people who have been murdered is greater than we think by far.

"It is going to be chilling."

FBI Director Louis Freeh has sent to Kosovo the agents that went to Kenya and Tanzania after two American embassies were bombed.

Based on their findings, a second, larger deployment will be flown to Kosovo in the next two weeks.

The experts will be trying to gather as much evidence as possible before winter sets in, burying the evidence under layers of mud and snow.

Nato has accused the Serbs of trying to cover up atrocities in Kosovo by exhuming some mass graves.

Emerging evidence

In Kacanik, the first evidence of Serb atrocities is reportedly emerging.

The UK's Press Association news agency reported that markers have been placed at two mass graves.

Indicted: Slobodan Milosevic

Sergeant Major Mick Robson, of the 1st Parachute Regiment, told PA: "When you look at what has happened in the villages you have to feel sorry for them.

"There are two mass graves in this village with an estimated 98 people in them."

The outgoing chief UN war crimes prosecutor, Louise Arbour, has asked for 300 forensic experts to investigation war crime allegations and find evidence.

In a letter to Mr Annan, she said experts need to search sites "preferably before the refugees return to their homes and villages" which will often be crime scenes.

She said: "We will only have one opportunity to complete these on-site investigations."

Arbour leaves post

Mrs Arbour: Delaying departure

A key figure under investigation will be Yugoslav President Slobodan Milosevic. He was indicted for crimes against humanity in May by Justice Arbour.

She has since been appointed to the Canadian Supreme Court, but Canadian Justice Minister Anne McLellan said Mrs Arbour would delay taking up her new post until 15 September to ensure "continuity in the work of the war crimes tribunal".

A new chief prosecutor will be chosen by the UN Security Council, which created the tribunal in 1993 to investigate war crimes in the former Yugoslavia.