Jan Pronk, the special UN envoy to Sudan, said on Tuesday arms were flooding into the troubled western region, violence was spreading beyond camps for the homeless, banditry was increasing and rebel groups were launching attacks in the area of oil facilities.
The renewed fighting comes just days after Khartoum signed a peace deal with rebels in the south.
"We may move into a period of intense violence unless swift action is taken," Pronk said.
"I do not exclude the possibility that the signature of the north-south agreement will be followed in the short term by an intensification of violence in and around Darfur."
Darfur has been caught up in an armed conflict since 2003, resulting in the death of some 70,000 people. About 1.7 million people have also been driven from their homes.
The crisis was sparked when rebel groups took up arms against the government in a struggle over power and resources.
Khartoum retaliated by arming nomadic militias.
Pronk suggested the talks on Darfur, held in Abuja, Nigeria, should concentrate on future political arrangements, including sharing power and wealth.
Security and humanitarian access should be excluded from Abuja and negotiated by a separate African Union ceasefire commission and others in Khartoum, he said.
Sudan's UN ambassador Elfatih Erwa agreed with Pronk on separating political talks from other issues, a move which might stop rebels from fighting the government.