Aid agency Medecins Sans Frontieres has halted its work in detention centres in a Libyan city because it said its medical staff were being asked to patch up detainees mid-way through torture sessions so they could go back for more abuse.
Rights groups have repeatedly raised concerns about torture being used against people, many of them sub-Saharan Africans, suspected of having fought for Muammar Gaddafi's forces during Libya's nine-month civil war.
The agency said it was in Misrata, about 200km east of the Libyan capital and scene of some of the fiercest battles in the conflict, to treat war-wounded detainees but was instead having to treat fresh wounds from torture.
"Patients were brought to us in the middle of interrogation for medical care, in order to make them fit for more interrogation," MSF General Director Christopher Stokes said in a statement.
"This is unacceptable. Our role is to provide medical care to war casualties and sick detainees, not to repeatedly treat the same patients between torture sessions."
The agency said it has raised the issue with the authorities in Misrata and with the national army.
"No action was taken," said Mr Stokes. "We have therefore come to the decision to suspend our medical activities in the detention centres."
Reports of the mistreatment and disappearances of suspected Gaddafi loyalists have embarrassed Libya's ruling National Transitional Council (NTC), which has promised to make a break with practices under Gaddafi and respect human rights.
The allegations are also awkward for the Western powers that backed the anti-Gaddafi rebellion and helped install Libya's new leaders.
The NTC has appealed to its citizens not to carry out reprisals against Gaddafi loyalists and it says it will investigate any abuses. There has been no comment yet from the NTC on the aid agency's allegations.
The ability of the government in Tripoli to rein in torture is limited because, in most cases, it is carried out by locally based militias who are outside the NTC's chain of command.
Human rights group Amnesty International said it had evidence of several detainees dying after being subjected to torture, including some in Misrata.
It quoted one man who said he had been tortured earlier this month in the headquarters of Misrata security forces.
"They took me for interrogation upstairs. Five men in plain clothes took turns beating and whipping me," Amnesty quoted the man as saying.
"They suspended me from the top of the door by my wrists for about an hour and kept beating me. They also kicked me."