The UN's humanitarian chief has called for an "immediate pause in hostilities" in Moadamiyah, a rebel-held suburb of the Syrian capital, Damascus.
Valerie Amos said 3000 people were evacuated last Sunday, but that the same number were trapped in the town amid continuing shelling and fighting.
Thousands of people were in similar situations in towns across the country, she said.
The BBC reports the army has previously said rebel-held areas must surrender or starve.
Also on Saturday, activists said at least 16 soldiers were killed in a suicide bombing and fighting at a checkpoint near the mainly-Christian area of Jaramana.
State media blamed "terrorists" for the explosion but did not give details.
Rebels control much of the countryside around Damascus but Jaramana - a Christian and Druze area mostly loyal to President Bashar al-Assad - is still held by the government.
In a statement, Baroness Amos said was "extremely worried by the situation unfolding across Syria where ordinary women, children and men are facing horrific violence and brutality from all sides of the conflict".
She said relief workers had been unable to access Moadamiyah for months, and that the ongoing conflict was preventing any rescue mission from going ahead.
"I call on all parties to agree an immediate pause in hostilities in Moadamiyah to allow humanitarian agencies unhindered access to evacuate the remaining civilians and deliver life-saving treatment and supplies in areas where fighting and shelling is ongoing," she said.
But she said this was not the only area of concern, citing Nubil, Zahra, old Aleppo town, old Homs town and Hassakeh as other places where people were in danger.
"Civilians must be allowed to move to safer areas without the fear of attack," she added.
Moadamiyah and at least two other Damascus's suburbs - Yarmouk and Eastern Ghouta - have been besieged by government forces for several months.
The situation has become so desperate that earlier this week Muslim clerics issued a religious ruling allowing people to eat cats, dogs and donkeys just to survive.
Those animals are usually considered unfit for human consumption in Islam.
Syrian activists say they are now starting to record the first deaths of complications caused by malnutrition.