KABUL, Afghanistan  An American military detention camp in Afghanistan is still holding inmates, sometimes for weeks at a time, without access to the International Committee of the Red Cross, according to human rights researchers and former detainees held at the site on the Bagram Air Base.

The site, known to detainees as the black jail, consists of individual windowless concrete cells, each illuminated by a single light bulb glowing 24 hours a day. In interviews, former detainees said that their only human contact was at twice-daily interrogation sessions.

“The black jail was the most dangerous and fearful place,” said Hamidullah, a spare-parts dealer in Kandahar who said he was detained there in June. “They don’t let the I.C.R.C. officials or any other civilians see or communicate with the people they keep there. Because I did not know what time it was, I did not know when to pray.”

The jail’s operation highlights a tension between President Obama’s goal to improve detention conditions that had drawn condemnation under the Bush administration and his stated desire to give military commanders leeway to operate. While Mr. Obama signed an order to eliminate so-called black sites run by the Central Intelligence Agency in January, it did not also close this jail, which is run by military Special Operations forces.