Hundreds of civilians are feared to have died in a surge of fighting between the Sudanese army and rebels in the turbulent Darfur region, a UN source told Reuters on Monday.
A Sudan army spokesman denied any fighting was taking place in Darfur's mountainous Jabel Marra region and accused insurgents of harassing and attacking locals.
Reports of clashes throughout last week have marred Khartoum's announcement of a new peace push in the region and come just over a month ahead of national elections.
"We think that we have a mounting number of casualties ... The lower estimate is around 140. The higher estimate is closer to 400," said a UN source, adding the figures referred to civilian deaths.
The source, speaking on condition of anonymity, said as many as 40,000 civilians had fled the fighting between the government and Sudan Liberation Army (SLA) rebels, 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 government bombing raids had killed at least 170 civilians around Deribat over the past 10 days and more had died in other areas.
The army spokesman told Reuters: "There are no clashes between the Sudanese army and the forces of Abdel Wahed's movement." Abdel Wahed Mohamed al-Nur is the leader of an SLA faction which has a stronghold in the area.
Darfur's conflict surged in 2003 when the SLA and other rebels took up arms against Sudan's government, accusing it of leaving the mostly desert region underdeveloped.
Sudanese president Omar Hassan al-Bashir declared the seven- year war over last Wednesday after reaching an initial settlement with the separate rebel Justice and Equality Movement (JEM), Darfur's most powerful rebel force.
Bashir's government signed an agreement in Doha on Tuesday committing Sudan to reaching a final peace deal with JEM by March 15.
But Abdel Wahed's SLA and other rebels have rejected the deal, demanding that security on the ground before talks.
JEM's negotiator in Doha, Ahmed Tugud, told Reuters it was unlikely the sides would meet the March 15 deadline as talks were currently stalled over plans for Sudan's government to sign a separate settlement with an umbrella group of small insurgent factions called the Liberation and Justice Movement.
Tugud said the plan, which he said was being proposed by international mediators, would give undue recognition to tiny rebel groups with little to no military presence on the ground.
"These are two persons pretending to be a rebel group, a man and his wife pretending to be a rebel group. It is rubbish ... We cannot have talks until we have solved this problem."
Darfur's joint UN African Union UNAMID peacekeeping force said it could not confirm the reports of the fighting because it did not have bases in the Jabel Marra area.