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GENEVA Nov 16 (Reuters) - All sides in Libya's conflict are committing breaches of international law that may amount to war crimes, including abductions, torture and the killing of civilians, the United Nations said on Monday.

Islamic State forces have gained and consolidated control over swathes of territory, "committing gross abuses including public summary executions of individuals based on their religion or political allegiance", said the joint report by the U.N. human rights office and U.N. Support Mission in Libya (UNSMIL).

Libyan armed groups pledging allegiance to the group control areas of central Libya including Sirte, Harawa and Nofliya, and have claimed responsibility for a number of attacks, including on oil fields, checkpoints and petrol stations, it said.

The U.N. report, referring to multiple armed conflicts in Libya, said: "All parties to the conflicts continue to commit violations of international human rights and humanitarian law, and abuses of human rights, including indiscriminate and disproportionate attacks; summary executions and other unlawful killings; arbitrary deprivations of liberty; and torture and other cruel, inhuman or degrading treatment."

Four years after the overthrow of Muammar Gaddafi, Libya is locked in a conflict between two rival governments -- an official one in the east and a self-declared one controlling Tripoli -- and the many armed factions that back them.

Thousands of people are being held in government detention centres and facilities run by armed groups amid "frequent reports of torture or other ill-treatment", the report said.

The warring factions have used imprecise weaponry in densely-populated residential areas in what has often amounted to indiscriminate attacks causing civilian deaths and damage to infrastructure, it added.

"UNSMIL has also received reports that air strikes by Operation Dignity, Libya Dawn and in one instance the Egyptian air-force led to civilian casualties and/or damage to civilian infrastructure," the report said. (Reporting by Tom Miles and Stephanie Nebehay; writing by Stephanie Nebehay; Editing by Gareth Jones)