The Red Cross interviews thousands of prisoners
A spokesman for the International Committee of the Red Cross in Geneva told BBC News Online that their staff had gained access to the Shibarghan detention centre two days ago and were now interviewing and registering some 3,000 prisoners.
"We cannot confirm the reports but we are talking to each and every one of the prisoners", Kim Gordon Bates said.
The New York Times newspaper has quoted a Northern Alliance commander in charge of the prisoners, General Jurabek, as saying that at least 43 Taleban fighters died of wounds or asphyxiation while being moved from the town of Kunduz to Shibarghan.
The paper said the number of dead may have been much higher, as the Northern Alliance transported hundreds of surrendered Taleban fighters - many of them foreigners - to Shibarghan - a journey that took two or three days.
The report quoted one detained Pakistani youth as saying all but seven people in his container had died from lack of air.
Another Pakistani said the survivors had taken turns to breathe through a hole in the container's metal wall.
Security concerns
The Red Cross spokesman said it had been difficult to gain access to the prison - the largest in northern Afghanistan - mainly for "security reasons".
He declined to comment on the conditions in the prison, saying only that the detainees were given "medical attention" by ICRC staff.
The ICRC is mandated under the Geneva conventions on the conduct of war to make sure prisoners of war are treated according to basic standards.
Among the thousands of prisoners at Shibarghan are thought to be about 80 who survived a prison revolt at a fortress near the city of Mazar-e-Sharif last month.
Hundreds of Taleban fighters died during the uprising, and the ICRC and other humanitarian organisations have expressed concern that many of the dead were killed after surrendering.