Other scientists questioned the accuracy of the refugee accounts and the reliability of the chemical analyses presented by the US government.

But human rights investigators said initial refugee accounts suggest that Drenica is emerging as a second area in Kosovo where particularly ruthless attacks are being carried out.

An American official said analysts were cross-checking satellite photographs with refugee accounts to piece together what happened.

From refugee accounts, Mr. Scheffer provided new details on the use of refugees as shields.

Units in Cambodia were composed of conscripts from the southern provinces of Vietnam, or, according to refugee accounts, of military misfits and "troublemakers."

Because they have been denied permission to travel around the country, United Nations investigations say they have not similarly been able to back up refugee accounts with these local witnesses.

State Department officials said the refugee accounts of atrocities by Serbs tended to support one another - in theme and often in graphic detail.

"If only two-thirds of the refugee accounts are true, this adds up to horrible events," Mr. Voorhoeve told reporters.

What they found confirmed - often with uncanny precision - widely reported refugee accounts of mass killings and other atrocities.

It was impossible to confirm the killings independently, beyond the refugee accounts, since reporters and independent investigators have been unable to visit that area of Kosovo since the bombing started.