Fighting has continued on Tuesday in the rebel-held Eastern Ghouta area in Syria, despite a five-hour "pause" ordered by the government's ally Russia.
Activists said the government carried out a number air and artillery strikes, while state media accused rebels of shelling a "humanitarian corridor" meant to allow civilians to leave.
As a result, there were no UN aid deliveries or medical evacuations.
Earlier, the US called on Russia to use its influence to secure a 30-day truce, reports BBC.
The UN Security Council unanimously passed a resolution demanding a nationwide cessation of hostilities on Saturday, but it did not give a specific start date.
A spokesman for the UN Office for the Co-ordination of Humanitarian Affairs (OCHA) said it had received reports that fighting continued after the Russian-ordered pause began on Tuesday morning.
"Clearly, the situation on the ground is not such that convoys can go in or medical evacuations can go out," Jens Laerke told reporters in Geneva.
The Syrian Observatory for Human Rights (SOHR), a UK-based monitoring group, said the situation in the Eastern Ghouta had been relatively calm overnight.
But it reported that in the first two hours of the Russian-ordered "humanitarian pause", a number of shells hit the towns of Douma, Harasta and Misraba.
Later, it reported air strikes, including the dropping of two barrel bombs on the town of al-Shifuniya, and rocket-fire in the enclave.