Fighting and shelling continued in eastern Ghouta, preventing any aid from reaching the besieged Syrian enclave during a five-hour pause "unilaterally" declared by Russia, the United Nations said.
"It is a question life and death - if ever there was a question of life and death - we need a 30-day cessation of hostilities in Syria as the Security Council demands," said Jens Laerke, spokesman of the UN Office for the Coordination of Humanitarian Assistance.
More than 1,000 sick and wounded are on a list of people needing medical evacuation by the Syrian Arab Red Crescent, World Health Organization spokesman Tarik Jasarevic said.
"But we don't have any updates on something like that taking place or coming right now," he added.
A five-hour ceasefire called by Russian President Vladimir Putin had been due to come into effect this morning after he ordered a daily "humanitarian pause" in the district.
The bombardment of Eastern Ghouta over the past week has been one of the heaviest of Syria's seven-year war, killing at least 556 people in eight days, according to a toll compiled by the Britain-based Syrian Observatory for Human Rights.
The Russian defence ministry said yesterday that the measures, decided in agreement with Syrian forces, were intended to help civilians leave and to evacuate the sick and wounded.
But the spokesman for Failaq al-Rahman, one of the main rebel groups in the Eastern Ghouta, accused Russia of presenting people with the choice of forced displacement or being killed in bombardment and siege, and called this a "Russian crime".
Eastern Ghouta is the last major stronghold near Damascus for rebels battling to topple President Bashar al-Assad, who has driven insurgents from numerous areas with military backing from Russia and Iran.
Fighting has escalated on several fronts in Syria this year. As Mr Assad has pressed the offensive against Eastern Ghouta, Turkey has launched an incursion against Kurdish fighters in the northwestern Afrin region.
Tension has also flared between Iran and Israel, which is deeply alarmed by Iran's expanding influence in Syria.
Syrian air defences shot down an Israeli F-16 earlier this month as it returned from a bombing raid on Iran-backed positions in Syria.
The Syrian war, which is approaching its eighth year, has killed hundreds of thousands of people and driven half of the country's pre-war population of 23 million people from their homes.