Some 7,000 people, many of them foreigners, are being held in Libyan detention centres controlled by ‘revolutionary brigade’ militias, according to a new UN report.

Women and children are among detainees and some have allegedly been tortured, said the report by UN Secretary General Ban Ki-moon ahead of a Security Council meeting on Monday on Libya since the death of dictator Muammar Gaddafi.

Political prisoners from the Gaddafi regime have been freed, but the estimated 7,000 new detainees have no access to courts "in the absence of a functioning police and judiciary," said the report.

The UN said sub-Saharan Africans accused of being Gaddafi mercenaries make many of those held.

"Some detainees have reportedly been subjected to torture or ill treatment. Cases have been reported of individuals being targeted because of the color of their skin," said Ban's report.

The largely tribal-based "revolutionary brigades" united to force Gaddafi out of power but have retained significant control in their own regions, according to diplomats and UN officials.

The report highlighted the town of Tawerga, whose inhabitants have been accused of siding with Gaddafi in the siege of nearby Misrata during the uprising.

"Tawergas are reported to have been targeted in revenge killings, or taken by armed men from their homes, checkpoints and hospitals and some allegedly later abused or executed in detention."

The UN report said the National Transitional Council (NTC) has started to take over from the militias, but "much remains to be done" to prevent abuse and improve detention conditions.

Ban said Libya's new leaders are "committed" to a society based on respect for human rights. But he added that it required "the earliest possible action" to halt "abuses and discrimination."

In a separate statement, Ban on Wednesday welcomed the announcement of Libya's interim government and again said it faces "numerous challenges," including "the crucial issues of national reconciliation, public security, human rights protection, and the resumption of basic services to the Libyan people."

Western governments that staged air strikes against Gaddafi targets have strongly supported the NTC and the new government, but most have cautioned the new authorities about human rights.

The United States' UN ambassador, Susan Rice, said she raised the issue in Tripoli this week with NTC leader Mustafa Abdel Jalil and incoming prime minister Abdel Rahim al-Kib.

Rice highlighted two "very sobering visits" she made in Tripoli.

One was the mass grave of victims of Gaddafi's forces near the feared Khamis 32nd Brigade headquarters. The second was the Sidi Bilal squatter camp for African migrant workers thrown out of their homes.

The workers from Nigeria, Ghana, Mali and other West African countries had been "traumatized, beaten, robbed, abused during the course of the violence," Rice told reporters.

The migrants do not know if they will be able to return home or get papers to stay and work in Libya.

"While we’ve seen really promising signs of progress, there are nonetheless reminders everywhere of Libya’s violent past in its remaining challenges including the crucial importance of protecting the rights of every Libyan and every person in Libya," Rice said.

UN envoy, Ian Martin, will brief a Security Council meeting on Monday about events since Gaddafi's killing in October.