Libyan militias are holding thousands of people in secret detention centres, while the interim government struggles to assert authority, the UN has heard.
The Security Council was told recent violence in Tripoli, Bani Walid and Benghazi highlighted the problem.
More than 8,000 pro-Gaddafi supporters are being held by militia groups, amid reports of torture, UN officials said.
The charity Medecins Sans Frontieres says it has suspended some operations because its work was being "exploited".
The humanitarian medical organisation said it had stopped work in detention centres in the north-western city of Misrata because some patients were being brought in for care between interrogation sessions.
At least four people died in clashes in Bani Walid, a former Gaddafi stronghold, on Monday.
The UN's Libya envoy, Ian Martin, told the Security Council in New York on Wednesday that those clashes between armed residents of Bani Walid and revolutionaries had been misreported as pro-Gaddafi forces retaking the city. Read More