The North African country is struggling with fighting on several fronts as brigades of former rebels who battled side by side to oust Muammar Gaddafi in 2011 now clash for political power and a share of oil revenues.
The conflict has driven at least 120,000 people from their homes and caused a humanitarian crisis, said a joint report by the U.N. human rights office and U.N. Support Mission in Libya (UNSMIL) that also documents shelling of civilian areas.
The toll includes an estimated 100 people killed in fighting between rival armed groups in Warshefana, near Tripoli, between late August and early October, and 170 killed in fighting in the Nafusa mountains to the southwest, it said.
Some 450 people have been killed in Benghazi since fighting escalated in mid-October. Hospitals in the city have been hit or occupied by armed groups, the report said.
At least 18 Libyan soldiers were killed and 69 others injured in clashes between forces loyal to former chief-of-staff Khalifa Haftar and militias in Benghazi, sources said Tuesday.
Medical sources at Al-Galaa hospital in Benghazi said that the hospital received 18 bodies of Libyan troops and 69 injured others after a day of clashes with Ansar al-Sharia militiamen.
Most of the fatalities were reported during clashes in Al-Sabri and Al-Lethi district in the city, they added.
"There is a serious lack of law and order, there is absolutely no accountability, so these violations are continuing with impunity, and there has been no effort to really stop that," U.N. human rights spokeswoman Ravina Shamdasani told a news briefing in Geneva on Tuesday.
"Some of these crimes may amount to war crimes," she said.
The International Criminal Court (ICC) is investigating the situation in Libya, but its chances of pursuing perpetrators are far from certain. In 12 years of operation it has secured just three convictions.
U.N. High Commissioner for Human Rights Zeid Ra'ad Al-Hussein said the threat of prosecution should, in any case, make militia leaders think twice about their conduct.
"As a commander of an armed group, you are criminally liable under international law if you commit or order the commission of grave human rights abuses or fail to take reasonable and necessary measures to prevent or punish their commission," he said.