The United Nations has said the horrific bloodshed in the Sudanese city of El-Fasher was a "stain" on the world, which has failed to halt the violence.
"Bloodstains on the ground in El-Fasher have been photographed from space," UN rights chief Volker Turk said, adding that "the stain on the record of the international community is less visible, but no less damaging".
"My staff are gathering evidence of violations that could be used in legal proceedings... (and) the International Criminal Court has indicated that it is following the situation closely," he said, adding that "all those involved in this conflict should know: We are watching you, and justice will prevail".
A special session on the situation in al-Fasher opened at the UN Human Rights Council in Geneva.
States will consider a draft resolution which requests a UN fact-finding mission to conduct an urgent inquiry into recent violations allegedly committed by the Rapid Support Forces and their allies in al-Fasher, as well as identifying the perpetrators.
In his opening address to delegates, the UN human rights chief urged the international community to act.
"There has been too much pretence and performance, and too little action. It must stand up against these atrocities - a display of naked cruelty used to subjugate and control an entire population," said Mr Turk.
The fall of al-Fasher on 26 October to the RSF cemented their control of the Darfur region in the more than two-and-a-half year civil war with the Sudanese army.
