Residents are reported to be fleeing the rebel-held Eastern Ghouta enclave in Syria as the situation there was described as "beyond critical".
Syria's military appears to be advancing on several fronts as it tries to retake the enclave just to the east of the capital, Damascus.
Government forces have now taken 25% of the area, UK-based Syrian Observatory for Human Rights monitors say.
A UN aid convoy planned for Sunday has not been able to enter the enclave.
The fighting since 18 February has left more than 600 people dead, many of them children.
How bad is the humanitarian situation?
Neither a daily five-hour truce ordered by the Russians - Syria's main backers - nor the ordering of a nationwide ceasefire by the UN Security Council have led to any humanitarian relief for the enclave.
The UN says that the "collective punishment of civilians is simply unacceptable".
The organisation said it had failed to obtain permission from Syrian officials to allow 40 trucks carrying humanitarian supplies to enter the key town of Douma.
Image copyright AFP Fighting in the Eastern Ghouta, Syria, 3 March 2018
Image caption More than 600 people have been killed in the past week in the Eastern Ghouta
"Instead of a much-needed reprieve, we continue to see more fighting, more death, and more disturbing reports of hunger and hospitals being bombed," UN regional humanitarian co-ordinator Panos Moumtzis said.
Some 393,000 people are trapped in the besieged enclave.