Hundreds of civilians are feared dead in the fighting, a UN source told the Reuters news agency on Monday.
"We think that we have a mounting number of casualties ... The lower estimate is around 140, the higher estimate is closer to 400," said the source of civilian deaths.
Speaking on condition of anonymity, the source said as many as 40,000 civilians had fled the fighting, most recently around the market town of Deribat.
"For us the important thing now is to get access to the area," said the source, who said aid workers and UN agencies had been blocked from entering Jabel Marra by the ongoing fighting and the threat of bandit attacks on their staff.
SLA field commander Suleiman Marajan told Reuters that government bombing raids had killed at least 170 civilians around Deribat over the past 10 days and more had died in other areas.
But a Sudan army spokesman denied any fighting in the mountainous Jabel Marra region and accused rebels of attacking local residents.
"There are no clashes between the Sudanese army and the forces of Abdel Wahid's movement," he told Reuters.
The reported clashes throughout last week have marred Khartoum's announcement of an end to war in the region and comes just over a month before national elections.
Omar Hassan al-Bashir, Sudan's president, declared the seven-year war in Darfur over last Wednesday after signing an initial deal with the Justice and Equality Movement (Jem), Darfur's main rebel force.
The deal signed in Qatari capital Doha on February 23 commits Khartoum to reaching a final peace deal with Jem by March 15.
State department spokesman Crowley said the peace deal offered a chance to reduce violence in Darfur but needed to be broadened to include other rebel groups, but Abdel Wahid's SLA and other rebels have rejected the deal, demanding security on the ground before talks.