Syrian government forces have killed more than 500 civilian during a week of intense bombardment of a rebel enclave near Damascus.
Activists say the victims in the Eastern Ghouta include 121 children, says the Syrian Observatory for Human Rights, a UK-based group monitoring the conflict.
Syrian government forces backed by Russia have been pounding the area since last Sunday.
The UN Security Council is struggling to agree on a ceasefire resolution.
A vote has been delayed several times since Thursday and is now due to be held later on Saturday.
On Saturday, the Syrian Observatory said at least 29 civilians were killed, including 17 in the main town, Douma, bringing the total to over 500 for the week.
The group said the strikes were being carried out by both Syrian and Russian planes, although Russia denies direct involvement.
Barrel bombs and shell fire have been dropped on the area, where some 393,000 people remain trapped.
Aid groups report several hospitals being put out of action since Sunday.
The Syrian government has denied targeting civilians and said it is trying to liberate the Eastern Ghouta from “terrorists” -a term it has used to describe both jihadist militants and the mainstream rebel groups that dominate the enclave.
The plight of civilians in the area has alarmed world leaders. Conditions there have been described as “hell on earth” by UN Secretary General, Antonio Guterres.
The Security Council has been debating a resolution calling for a 30-day calm to allow for aid deliveries.
But Russia wants changes. Under current draft, any ceasefire would not apply to the Islamic State group, or the Nusra Front – formerly al-Qaeda’s official affiliate in Syria.
Moscow says it must go further and exclude other groups “co-operating with them” and which have shelled Damascus.
These could include the two biggest rebel groups in Eastern Ghouta – Jaish al-Islam and Faylaq al-Rahman. Faylaq al-Rahman has in the past fought alongside the jihadist group Hayat Tahrir al-Sham, an alliance of factions led by the Nusra Front.
Western diplomats have accused Russia of stalling for time.
They suspect that Moscow wants to give Syria time to deal a final blow to forces defending the enclave on the edge of Damascus.
On Friday evening US President Donald Trump pointed the finger of blame firmly at Syria and its allies, Russia and Iran, for the humanitarian crisis.