"A multitude of actors, both state and non-state, are accused of very serious violations and abuses that may, in many cases, amount to war crimes," UN human rights chief Zeid Ra'ad al-Hussein said in a statement.
A report released on Thursday documenting abuses committed in Libya between 2014 and 2015 warned the situation had deteriorated dramatically.
Libya has had rival administrations since the summer of 2014 when the recognised government fled Tripoli after the Fajr Libya militia alliance overran the capital.
A power vacuum since the 2011 toppling of Muammar Gaddafi has fostered the rise of the Islamic State group in the country, which is currently headquartered in the former dictator's hometown of Sirte, but control of Benghazi remains divided between a collection of militias.
Thursday's report details how most major armed groups in the country "have carried out unlawful killings", mainly executing people they had taken captive and assassinating those seen voicing dissent.
| Most major armed groups in the country have carried out unlawful killings, mainly executing people they had taken captive and assassinating those seen voicing dissent |
The six-member investigation team behind the report was only able to visit Libya briefly during their year-long probe due to the dire security situation but conducted interviews with more than 200 victims and witnesses.
They described how bodies of people detained by different armed groups and the Libyan National Army, were found bearing clear evidence of torture and execution.
Repeatedly raped
The report also details widespread and horrific torture by different factions, including beatings with plastic pipes and electrical cables, prolonged suspension in stress positions, electric shock and forced starvation. According to the report, a number of detainees had been tortured to death.
The report, which was ordered by the UN Human Rights Council, said thousands of people were currently in detention, most without any proper examination of their cases, and many in secret detention centres.
Sexual violence, while difficult to document, appears to be rife in places of detention, the investigators said.
They detailed one case where a woman said she had been abducted in Tripoli by members of an unidentified armed group, before being drugged and repeatedly raped over six months. After her escape, she discovered she was pregnant.
The woman said she had seen six girls as young as 11 who were sexually abused by members of the same group.
| The report detailed one case where a woman said she had been abducted in Tripoli by members of an unidentified armed group, before being drugged and repeatedly raped over six months |
The investigators also decried the forcible recruitment of children by armed groups, including those linked to the Islamic State, who celebrated the practice.
The report cited two boys, aged 10 and 14, who said they had been forced from their families to take part in IS weapons training, using live ammunition, and watch videos of beheadings.
"One of the most striking elements of this report lies in the complete impunity which continues to prevail in Libya and the systematic failures of the justice system," Zeid said.
Thursday's report called on the Security Council to list the "individuals responsible for violations or abuses" and slap them with sanctions.
They also complained that the International Criminal Court tasked with investigating the abuses did not have the resources to do so properly.
The UN human rights chief also called on the Security Council to increase monitoring in Libya.