German Chancellor Angela Merkel has called for an end to the "massacre" in Eastern Ghouta after fresh air strikes on the rebel-held enclave killed another 13 people.
Condemning the Syrian regime for targeting "its own people", Ms Merkel said: "The killing of children, the destruction of hospitals - all that amounts to a massacre that must be condemned and which must be countered with a clear no".
At least 335 civilians have been killed since Sunday in the besieged enclave of Eastern Ghouta near Damascus as regime forces intensified their bombardment of the site.
The German leader's comments came on the eve of the UN Security Council's expected meeting in New York to vote on a resolution calling for a 30-day ceasefire, the delivery of aid, and the evacuation of the wounded.
Sweden and Kuwait, which drafted the measure, requested the vote "as soon as possible," the Swedish mission said.
It remained unclear whether Russia would resort to its veto to block the draft resolution.
The Kremlin said yesterday that Russia was not responsible for the civilian deaths in Eastern Ghouta and said it was working with the regime to fight terrorism.
"Those who support the terrorists are responsible," said Dmitry Peskov, a Kremlin spokesman. "Neither Russia, nor Syria, nor Iran are in that category of states as they are waging an absolute war against terrorists in Syria."
Both Russia and China have previously blocked UN resolutions critical of the Assad regime.
Staffan de Mistura, the UN special envoy on Syria, said yesterday he hoped the Security Council would agree a resolution to end fighting in Eastern Ghouta, but said it would not be easy.
"I hope it will. But it's uphill. But I hope it will. It is very urgent," he said as he arrived at the United Nations in Geneva.
Ms Merkel's intervention followed a statement from the White House saying the US "strongly condemns recent attacks on the people of Syria in Eastern Ghouta by Russia and the Assad regime".
"We fully support the call from the United Nations for a cessation of violence to allow for the unfettered delivery of humanitarian supplies and urgently needed medical evacuations of civilians," the White House said.
The Syrian Observatory for Human Rights said at least 13 people were killed in Douma, the main town in Eastern Ghouta, three of them children.
"It was raining on the area so there are no warplanes but it seems the regime chose to use rockets instead," Rami Abdel Rahman, the head of the Britain-based war monitor, said.
"The rocket fire hasn't stopped this morning. Around 200 ground-to-ground rockets struck Douma alone," he said, adding that the number of wounded across the enclave yesterday topped 120.
According to state media and the Observatory, at least 15 people have been killed by mortar rounds and rockets fired in the other direction, by Islamist and jihadist groups, over the same period.
Residents of Douma, the biggest town in the district, described plumes of black smoke billowing from residential areas after planes dropped bombs from high altitude.
Searches were under way for bodies amid the rubble in the town of Saqba and elsewhere, said rescuers.