A UN team is due to arrive in Tripoli to investigate allegations of human rights violations in Libya since the start of the conflict in February.
The team was appointed by the UN Human Rights Council following the Libyan government's crackdown on protesters.The government has said it will co-operate with the inquiry.
The three investigators say they will look at all alleged abuses, including those the government says have been committed by rebels or Nato forces.
The original mandate - to examine human rights violations allegedly committed by the forces of Libyan leader Col Muammar Gaddafi - remains the priority, says the CA's Imogen Foulkes in Geneva, where the UN Human Rights Council is based.
There have been reliable reports of enforced disappearances, torture and even killing of protesters, says our correspondent.
The UN human rights commissioner, Navi Pillay, said in late February that what was happening in Libya "may amount to crimes against humanity".
More recently, there have been reports that Col Gaddafi's forces trying to retake Misrata from rebels are indiscriminately shelling the city.
On Tuesday, three people were reportedly killed as missiles slammed into the city's port, a lifeline for those seeking to escape to the rebel stronghold of Benghazi.
Misrata has been besieged by government forces for two months, leaving parts of the city with neither electricity nor water.
Continued sniper fire, street clashes and shelling have prevented people from venturing outside their homes to get food and medicine.
Human rights groups say more than 1,000 people have been killed in the fighting and many more have been wounded. Ships have been ferrying the injured to hospitals in Benghazi and bringing in humanitarian aid.
Libya's government denies it has been indiscriminately shelling civilian areas.
Misrata is the last major rebel-held city in western Libya and the fighting for it has been fierce.
The UN investigators are to present their findings to the Human Rights Council in June. But their work could be overtaken by other moves, says our Geneva correspondent.
The UN Security Council has asked the International Criminal Court to investigate Libya on possible charges of war crimes.
'Back foot'
Nato is enforcing a UN resolution to protect civilians in Libya amid a two-month revolt inspired by other uprisings in the Arab world.
A recent Nato strike on Col Gaddafi's compound in Tripoli sparked angry criticism from Russian Prime Minister Vladimir Putin, who said the Western coalition had no mandate to kill the Libyan leader.
US Defence Secretary Robert Gates defended the strike, calling it a legitimate attack on a military command and control centre.He spoke after a meeting in Washington with British Defence Secretary Liam Fox, who said Libya's rebels had gained "momentum" on the battlefield and that Col Gaddafi's forces were on the "back foot".
British Foreign Secretary William Hague said Nato's 1,500 strike sorties over Libya had "seriously degraded Gaddafi's military assets and prevented widespread massacres planned by Gaddafi's forces".
"They remain unable to enter Benghazi and it is highly likely that without these efforts Misrata would have fallen, with terrible consequences for that city's brave inhabitants."
On Tuesday, the US eased oil sanctions against Libya.
The move allows rebels to sell oil within their control and US firms to engage in transactions involving oil and oil products, and natural gas, as long as the exports benefit the opposition Transitional National Council.
