(CNN) -- A Libyan diplomat died 24 hours after he was detained by a militia based in the city of Zintan, Human Rights Watch said.
The French Foreign Ministry confirmed Omar Brebesh's death in prison, though it did not have any information as to the circumstances. The ministry said Brebesh, 62, had served as the charge d'affairs in France from 2004 to 2008.
CNN's attempts to reach Libyan officials were not immediately successful. However, Libya's ambassador to the United Nations, Mohammed Shalgham, told the United Nations this month that Libya does not approve of any abuse and was working to stop any such practices.
Brebesh was detained January 19 and appears to have died from torture, Human Rights Watch said Thursday after viewing a preliminary autopsy report.
The autopsy report said the cause of death included multiple bodily injuries and fractured ribs. Photos of Brebesh's body, seen by Human Rights Watch, show welts, cuts, and the apparent removal of toenails.
The rights monitoring group said it also read a Tripoli police report that said Brebesh had died from torture and that an unnamed suspect had confessed to killing him.
His death comes amid various reports of detainee abuse and sharp criticism that Libya's new leaders have failed to establish the rule of law.
Amnesty International said this month that several detainees have died after being tortured in recent weeks. And the medical charity Doctors Without Borders said it was halting its work in detention centers in Misrata because detainees were tortured and were denied urgent medical care.
Human Rights Watch said Libya's militias will continue torture and abuse unless they are held to account.
"Libya's leaders should show the political will to prosecute people who commit serious crimes, regardless of their role in the uprising," said Sarah Leah Whitson, the group's Middle East and North Africa director.
"The rule of law, and punishment for crimes, apply to all Libyans, including those who fought against Moammar Gadhafi," she said.
Brebesh's son Ziad told Human Rights Watch that his father voluntarily submitted to an investigation by the Al-Shohada Ashura militia at their base in the Tripoli neighborhood of Crimea. Brebesh had been called there for questioning.
Ziad escorted his father, who entered the base at 5:30 p.m. January 19. Ziad said he stayed inside for tea before being told to wait outside.
After 45 minutes, militia members took Ziad away to retrieve one of the family cars and a firearm. He returned later that night but was prevented from entering the area where his father was being interrogated, Human Rights Watch said.
The next day, following a visit to the Al-Shohada Ashura base, the family heard that Brebesh's body had appeared at a hospital in Zintan, about 100 kilometers southwest of Tripoli. Ziad's brother Muhammad went there in the evening and described what he saw:
"I saw his face. There was blood on his nose and mouth. But I didn't see the rest of his body or his face from the other side. There was a bump on his forehead. After that, I kissed him and that was it. Later, when we saw the other side of his face at the hospital in Tripoli, it looked like his jaw was broken, like his face was not in the right place."
Brebesh's family showed photographs of his body to Human Rights Watch that revealed welts and extensive bruising on the abdomen, lacerations on both legs, and a large wound on the sole of the left foot. Some of his toenails appear to have been removed, the rights group said.