The United Nations says Darfur refugees are being systematically starved. Human Rights Watch says black Africans are deliberately being driven off the land.
The UN report said a team found “appalling and outrageous” conditions when it visited the town of Kailek less than two weeks ago.
It said hundreds of refugees were living amid their own piles of excrement, and were being deliberately starved by Arab militia occupying the destroyed town.
Up to 1,700 internally displaced people (IDPs) were kept under “appalling” and “outrageous” conditions for months in Kailek, a looted town where almost no building has been left intact following attacks by government troops and Arab militia.
The report said women and girls in Kailek spoke of their fear of being abducted in the night and raped by the Janjaweed militia.
“With less than minimum water supplies available, the hygienic conditions of both the location itself as well as its inhabitants are outrageous,” the report said.
Militias prevented food deliveries and stopped anyone leaving, the report said.
Sudanese Foreign Minster Mustafa Ismail has acknowledged there might have been human rights violations, but denies a campaign of ethnic cleansing.
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.
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, according to 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.
The local authorities in Kass have denied they colluded in the siege, but survivors have told 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.
The Janjaweed militia is accused of holding towns hostage.
A report released by the New York-based Human Rights Watch has also accused the Sudanese government of ethnic cleansing and crimes against humanity in the Darfur region.
It said 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.