* U.N. rights chief cites lack of oversight
* 8,000 prisoners in Tripoli alone - ambassador
* Unauthorized detention centers condemned
UNITED NATIONS, Jan 25 (Reuters) - Detainees from Libya's civil war held by revolutionary brigades continue to be subjected to torture despite efforts by the provisional government to address the issue, the U.N. human rights chief said on Wednesday.
Navi Pillay told the U.N. Security Council she was extremely concerned about thousands of prisoners, most of them accused of being loyalists of the toppled government of Muammar Gaddafi and many from sub-Saharan Africa.
"The lack of oversight by the central authorities creates an environment conducive to torture and ill-treatment," Pillay said. "My staff have received alarming reports that this is happening in places of detention that they have visited."
She said it was urgent that all Libya's detention centers be brought under control of the Ministry of Justice and General Prosecutor's Office and that detainees be screened so that they could be freed or receive a fair trial.
The government that replaced Gaddafi's administration has been struggling to take control of the detainees held by the revolutionary brigades who did the fighting, but has been hampered by lack of prison staff, U.N. officials say.
Ian Martin, the U.N. special envoy for Libya, told Wednesday's council meeting that the Justice Ministry had so far taken over six prisons from the revolutionary brigades.
Neither Pillay nor Martin gave any figures for the number of people held by the brigades. U.N. Secretary-General Ban Ki-moon said in a report last November that it was around 7,000.
Libyan Ambassador Abdurrahman Mohamed Shalgham told the council there were more than 8,000 prisoners in Tripoli alone but did not make clear if that included people held by the authorities. He said his government condemned the use of unauthorized detention centers.
"We have spoken to our brothers and we have said, 'Any individual who has not committed a crime, or who has not participated in massacres will have their passports,'" he said.
On allegations that NATO caused civilian casualties during a bombing campaign that helped the rebels overthrow Gaddafi last year, Pillay called on the alliance and other parties to cooperate with a Libya commission of inquiry set up by the U.N. Human Rights Council.
"Information so far indicates that NATO made efforts to keep civilian casualties at a minimum, but where civilians have been killed and injured, the alliance should disclose information about all such events and about remedial actions undertaken," she said.
Shalgham said Libya was aware of four incidents, but questioned whether NATO was at fault in any of them.
"Without NATO, hundreds of thousands of people would have died in Benghazi," the birthplace of the uprising, he said.
Reporting By Patrick Worsnip; Editing by Xavier Briand