Thousands of people are trapped in Sudan's Darfur region, unable to cross into Chad, amid a government offensive, rebels and aid workers say.
More than two million have been displaced by the Darfur conflict
Aid agencies say the civilians took refuge in the Jebel Moun mountains following government bombing of three rebel-held towns earlier this month.
The rebels say Sudanese troops are stopping civilians crossing the border.
Sudan's Foreign Ministry told the Associated Press (AP) news agency it was unaware of any stranded refugees.
At least 8,000 refugees have been trapped in the area, unable to flee into Chad, UN spokeswoman Orla Clinton told the Associated Press news agency.
Sudanese forces say they have taken control of Jebel Moun and inflicted heavy casualties on the rebels of the Justice and Equality Movement (Jem).
Jem accused the government of indiscriminately bombing civilians and killing 15 people.
UN Secretary General Ban Ki-moon earlier condemned the bombing of a camp for displaced people by Sudanese government aircraft at Jebel Moun on Tuesday.
The camp was empty at the time. Eyewitnesses and rebels said army helicopters, Antonov aircraft and state-backed militias had carried out a three-pronged offensive.
Aid agencies were already struggling to cope with a wave of around 12,000 people who fled Darfur into Chad after bombing raids earlier this month.
More than 100 people were killed in that offensive against three major towns, residents say.
At least 200,000 people have died in five years of conflict between rebels, the army and pro-government militias in Darfur.
France has urged the Sudanese authorities to ensure immediate free access to the area for humanitarian workers.
The BBC's Amber Henshaw in Darfur says Jebel Moun remains out of bounds to aid workers and the UN peacekeeping mission to Darfur, Unamid, raising major concerns about thousands of people living in the area.
Unamid began deploying in January but the force still lacks most of the 26,000 personnel planned for the mission.