The SLA insists on the disarmament of Arab militias before talks
Renewed fighting is reported to have broken out in Darfur despite Sudan's President Omar al-Bashir declaring the war in the western region was over.
French aid group Medecins du Monde said it has had to suspend its operations in central Darfur's Jebel Marra.
A faction of the rebel Sudan Liberation Army (SLA) said there had been heavy fighting late into Wednesday night.
The SLA did not agree to the ceasefire that Khartoum reached with the region's other main rebel group, Jem, this week.
A spokesman for the SLA faction loyal to Abdel Wahid Mohamed el-Nur said government forces attacked several positions in its mountainous stronghold, including the town of Deribat.
"The government attacked in huge numbers backed up by Antonovs, helicopter gunships and MiGs [Russian military aircraft]," Ibrahim al-Hillu told Reuters news agency.
The agency reports that the army denied there have been clashes.
Medecins du Monde said it was concerned about those displaced in the fighting, but did not specify who was involved.
"Deribat, a town of 50,000 residents, was attacked on Wednesday, forcing the population to flee and bringing the number of displaced in the area to 100 000," the aid group said in a statement.
SLA-Abdel Wahid controls part of the Jebel Marra and has wide support among the Fur ethnic group in Darfur's massive camps for those displaced in the seven-year conflict.
Mr Wahid, who lives in exile in Paris, has refused to enter into peace talks until there is security in the region and the Arab Janjaweed militias are disarmed.
Mr Bashir is still subject to an international arrest warrant for war crimes in Darfur, over claims he supported Arab militias who massacred black African communities.
The ceasefire agreement with the Justice and Equality Movement (Jem) was signed after a year of on-off negotiations.
A final peace accord is expected next month but Jem leader Khalil Ibrahim has again called for the postponement of elections due to be held in April.
"Our position is to ask for a delay in these elections because there are numerous citizens of Darfur and Kordofan who will not be able to take part because they are displaced," AFP news agency quotes him as saying.
Under the deal, Jem rebels will be given a share of power "at all levels" in Sudan.