"Ian Martin, head of the UN mission to Libya, said good progress was being made toward the country's first democratic election, but militia prisons were one of a number of "serious obstacles" to establishing the rule of law.
"Cases of mistreatment and torture of detainees continue," Martin told the UN Security Council. The UN has raised "deep concern" over the deaths in April of three people at a prison in Misrata which comes under government authority.
There was "credible information" that the deaths were caused by torture and that at least seven other people had been tortured at the same prison, said the head of the UN Support Mission in Libya, or UNSMIL.
There have also been allegations of torture at prisons in Tripoli, Zawiya and Zintan.
The UN said in October that 7,000 prisoners, mainly supporters of Muammar Gaddafi, were held by the revolutionary brigades which led the fight to overthrown the late leader last year."