Andrew McEntee said that "very credible evidence" in a British documentary film needed to be investigated. He was speaking after the first showing in Berlin of the film, Massacre at Mazar.

"This film raises questions that will not go away," said Mr McEntee, who led Amnesty International UK in the 1990s and is now an international human rights lawyer.

The documentary describes how thousands of Taliban troops were rounded up after the battle of Kunduz in late November and transported in sealed shipping containers to Sheberghan prison, a jail then under US control in northwestern Afghanistan.

The film alleges that large numbers of the prisoners died during the journey. US troops suggested the drivers take the bodies out into the desert at Dasht-i-Leili for burial. Two men said they were forced to drive hundreds of Taliban, many of whom were still alive, into the desert, and said that the living were shot. Footage showed large areas of compact red sand dotted with the traces of bones, including jaw bones, and pieces of clothing.

The filmmakers claim that thousands of Afghans, Pakistanis, Uzbeks, Chechens and Tajiks may now be buried at the mass grave. UN and human rights officials have found the grave but have not estimated the number it contains. Only 15 bodies have been excavated.

A Pentagon spokesman last night denied the allegations: "US Central Command looked into it a few months ago, when allegations first surfaced when there were graves discovered in the area of Sherberghan prison. They looked into it and did not substantiate any knowledge, presence or participation of US service members."