Militia attacks have forced aid workers to evacuate, closed off roads, and sent 7000 Darfuris from their homes in South Darfur and West Darfur, Radhia Achouri, a UN spokeswoman, said on Friday.
"Our humanitarian efforts are being destroyed on the ground," she said.
One of the world's largest humanitarian aid operations is under way in Darfur, with more than 11,000 aid workers trying to feed, clothe and shelter the more than two million Darfuris who fled to miserable camps during almost three years of fighting.
A 6000-strong African Union force deployed to monitor the violence secured a brief respite but recent months saw a return to insecurity.
Achouri said government forces and Darfur rebels have been fighting in West Darfur, and there have been joint army and militia attacks on villages in South Darfur.
In West Darfur there are also reports of Arab militias fighting each other as their traditional nomadic migratory paths have been blocked by the conflict and desertification has dried up many of the drinking holes for their animals.
Talks to halt the violence in
Darfur have resumed in Nigeria
Water points have been targeted, making it difficult for civilians to return home.
Meanwhile, all roads out of the West Darfur state capital el-Geneina have been closed to UN traffic.
Now many aid workers have been temporarily evacuated from two main areas of operations and a rebel renegade group is threatening helicopters, prompting the UN to ground its aircraft over their areas, UN officials said.
"Humanitarian access is worse than ever," said UN worker Matthew Ryder in el-Geneina.
Meanwhile delegates at the AU-sponsored peace talks seeking an end to the 33-month-old crisis in Sudan's Darfur region returned to the negotiating table in a bid to resolve a row over power and wealth sharing, an AU spokesman said.
"The meeting of the commission on power sharing is now under way. While that of wealth sharing will start at 4.00pm," Nouredine Mezni said.
"The representatives of the government of Sudan and those of the movements are expected to respond to a compromise proposal by the AU," he said.
"Our humanitarian efforts are being destroyed on the ground"
Radhia Achouri, UN spokeswoman
Mezni added that the meeting on power sharing was postponed to allow the region's two rebel movements to reach a compromise with delegates from the Khartoum government on some contentious issues.
The row over power sharing had almost stalled the latest round of talks which resumed on 28 November.
Up to 300,000 people have died and more than two million fled their homes in what UN aid agencies have dubbed world's worst humanitarian crisis.
The UN has accused Khartoum of arming Arab proxy militias to fight the rebels who say they are marginalised by the central Arab-dominated government.