Two reports have accused the Sudanese government and Arab militia of massive abuses in western Darfur region.
Some one million people have been displaced by fighting
The United Nations says Darfur refugees are systematically being starved. Human Rights Watch says black Africans are deliberately being driven off the land.
In a report published on Friday, the UN Commissioner for Human Rights described a "reign of terror" in the region.
It said a UN team found "appalling" conditions when it visited the town of Kailek less than two weeks ago.
Militias prevented food deliveries and stopped anyone leaving, the report said.
Sudanese Foreign Minister Mustafa Ismail has acknowledged there might have been human rights violations, but denies a campaign of ethnic cleansing is going on.
Later on Friday, the report will be presented to the UN Security Council in New York.
One aid worker in Kailek described what happened there as the "politics of starvation".
Eight or nine children were reported to have been dying from malnutrition every day.
The report said women and girls were raped and described inhumane sanitary conditions and a lack of medical treatment.
Members of the UN team were said to be "visibly shaken" by circumstances in the town.
Refugees first sought shelter in Kailek, the biggest settlement in the area, after Arab militias started attacking nearby villages.
The horseback militia known as the Janjaweed surrounded the village, effectively holding 1,700 people hostage.
The UN report says that as food began to run out, residents were forced to start paying the militia to leave the village to look for supplies.
The refugees were later moved to a nearby town, Kass, in advance of a visit by another UN team.
The survivors in Kass have been camping in a disused secondary school.
They appeared dazed and traumatised, says the BBC's Ishbel Matheson.
One three-year-old girl lying on open ground was little more than a skeleton. There was no medicine to treat her and very little food, our correspondent adds.
The local authorities in Kass deny that they colluded in the siege, but survivors tell a different story.
They are adamant that they saw government forces working alongside the militia.
The UN report said it believed there were other villages like Kailek in western Sudan, where civilians were living in similar conditions.
A report released by the New York-based Human Rights Watch has accused the Sudanese government of ethnic cleansing and crimes against humanity in the Darfur region of western Sudan.
The Janjaweed militia is accused of holding a town hostage
It says the tactics involved include mass rape, summary execution and the systematic burning of villages and crops.
Meanwhile, the foreign ministry in Chad has summoned Sudan's ambassador to protest against cross-border incursions by militia from Darfur.
The Chadian government has urged Sudan to assert control over the militia, following the latest clash on Wednesday.