Government forces have counter-attacked a rebel-held town in southern Darfur, the insurgents said on Tuesday as the UN's top Sudan official called for immediate access to help thousands of civilians.
"This morning we resisted a government attack on Labado," Nuraldim Mohammed Taha, London representative of the Sudan Liberation Army's Minni Minnawi faction, told AFP from London.
The insurgents 10 days ago began an occupation of Labado and Muhagiriya, strategically located communities about 100 kilometres (60 miles) east of the South Darfur state capital Nyala, Sudan's second city.
Their occupation is unusual as rebels normally stage hit-and-run attacks.
Sudan's Defence Minister Abdelrahim Mohammed Hussein vowed last week to move against the insurgents.
Authorities have deemed the area too unsafe for aid agencies to enter but "the situation does seem to be changing rapidly on the ground," said Mark Cutts, head of the UN's Office for the Coordination of Humanitarian Affairs in Sudan.
Ali Al-Za'tari, the UN's chief in Khartoum, said government figures show that about 36,000 people have gathered around the bases of international peacekeepers in Muhagiriya and Labado.
"Reports received by the United Nations indicate an outbreak of diarrhoea among children requiring urgent and immediate assistance," Za'tari said, expressing "serious concern" for civilians in the area.
He said aid workers need access right away to assess the people's condition and provide relief supplies.
Za'tari urged the government and Minnawi rebels "to uphold their humanitarian responsibilities to protect all civilian populations."
He stressed that civilians wanting to leave the area must be allowed safe passage.
"All the normal services have been disrupted," added Cutts. "Too many civilians have already suffered in Darfur. We need this fighting to stop."
Minnawi and other ethnic rebels in Sudan's far-western Darfur region rose up against the Arab-dominated Khartoum government in 2003.
Rebel splinter factions signed a 2011 peace deal with the government but Minnawi and other major insurgent groups rejected the agreement.
Cutts told AFP the latest fighting "is obviously a setback for all those who are working to build peace and put Darfur back on the road to recovery and development."
But he said parts of the region, which is about the size of France, are "relatively stable" and offer good opportunities for rebuilding.
A conference in the Gulf state of Qatar last week backed a six-year strategy aiming to move Darfur away from food handouts and other emergency aid, and laying the foundation for lasting development.
However, Canada told the meeting that it faced "unacceptable obstructions to our development and stabilisation efforts" in Darfur, where security has "worsened."
There have been "frequent delays and refusals" of travel permits, Canada said.
At the same conference the European Union said aid agencies still face "access restrictions that compromise donors' ability to plan and monitor their programmes".
Khartoum has recently taken measures it says should make it easier for aid workers to visit their projects.
"The government commitment has been to allow us to move freely in all these places where there are not ongoing military operations," Cutts said.
Separately the Salamat tribe accused members of the paramilitary Central Reserve Police of joining recent fighting in Rahad el Berdi near Umm Dukhun in Darfur's southwest.
A Salamat statement said dozens had died in Rahad el Berdi and Umm Dukhun.
At least 18 people were killed in fighting between the Misseriya and Salamat tribes around Umm Dukhun, a tribal leader said last week.