Civilians in South Sudan have been the main target of recent ethnic violence that is likely to have killed thousands, and thousands more have been raped, arrested and tortured, the United Nations said.
The UN said the report on the first six weeks of the conflict offered a "snapshot" of violence perpetrated mainly by forces loyal to President Salva Kiir and by those of his opponent former vice president Riek Machar.
It is the first report on human rights abuses committed during the conflict.
Kiir is from the Dinka tribe, the country’s largest and most powerful, while Machar is an ethnic Nuer.
During more than 500 interviews, the report said, witnesses, victims and government and security officials reported the deliberate targeting of civilians.
Investigators looked at developments between December 15, when the conflict began, and January 31, the Associated Press news agency reported.
The UN said that those targeted included nationals and foreigners slain in "extrajudicial and other unlawful killings, including mass killings, enforced disappearances, gender-based violence such as rapes and gang rapes, and instances of ill-treatment and torture by forces from both sides of the conflict."
Human rights experts said "it is premature to judge whether or not sexual violence was used as a weapon of war" since incidents are still being investigated.
The interim report focuses on alleged rights abuses in the four states which have seen the heaviest fighting - Central Equatoria where the violence began in the capital Juba on December 15, and Jonglei and oil-rich Unity and Upper Nile states where it quickly spread in the following days.
The government insists that the unrest was sparked by a failed military coup mounted by soldiers loyal to Machar. It arrested 11 officials it suspected of involvement in the coup plot but seven have since been freed as part of a ceasefire agreement signed last month.
Machar denies the coup allegation but says his goal is to have Kiir, who is backed by troops from neighbouring Uganda, removed from power.
While the trigger for the violence remains in dispute, the report said it has led to "a major security, human rights, and humanitarian catastrophe," and increased ethnic polarisation in the world's newest nation.
The report does not cover events in February but it said the situation on the ground is still volatile, and violations of human rights are continuing, especially in areas where there is continued fighting.
The UN mission, in a release accompanying the report, said that in the recent battle for control of Malakal, the capital of Upper Nile State, there was fresh evidence of rights abuses including the execution of two children outside the perimeter of the UN compound on Thursday.
During the first six weeks of the conflict, the report said, "it is clear that civilians bore the brunt of much of the fighting and that gross violations of human rights were committed".