Doctors Without Borders pleaded on Monday for access to treat the wounded in the rebel-held part of Syria’s Aleppo as government forces pressed ahead with an offensive that has killed hundreds of people in recent weeks.

The international charity, also known by its French acronym MSF, said in a statement that medical workers in Aleppo are exhausted and that the overstretched facilities face an impending fuel shortage. MSF, which supports eight hospitals in Aleppo’s besieged eastern quarters, says just 35 doctors remain in the area, serving a population of 275,000.

Eastern Aleppo’s Health Directorate said the wounded were sleeping outside overcrowded hospitals, waiting for care. The U.N. has warned that the Aleppo bombardment by Syrian and Russian warplanes could leave thousands more dead by the year’s end.

“Russia and Syria must stop the indiscriminate bombing now and abide by the rules of war to avoid the extreme suffering of the unprotected civilian population,” said Pablo Marco, MSF’s operations manager for the Middle East.