Russia has said there was no agreement at the UN Security Council on plans to introduce a 30-day national ceasefire in Syria.
The deadly bombardment of eastern Ghouta by Syrian Government forces is said to have killed at least 37 more civilians early on Thursday.
The UK-based Syrian Observatory for Human Rights said 37 civilians had been killed by Thursday afternoon and up to 416 people, including 96 children and 61 women, have died from "concentrated fire" since Sunday night.
The UN said on Wednesday that at least 346 people had been killed and hundreds more wounded since President Assad's forces stepped up their offensive at the start of the month.
Hell on earth
At least 92 people died in just one 13-hour period on Monday, the UN added, with Secretary General Antonio Guterres calling it "hell on Earth" and urging an immediate suspension of "all war activities."
On Thursday, German Chancellor Angela Merkel added to the international outcry, calling the situation a "massacre."
She said the Syrian regime was "fighting not against terrorists, but against its own people; the killing of children, the destruction of hospitals, all this is a massacre which needs to be condemned."
Around 400,000 people are living under siege in eastern Ghouta, near the capital Damascus, with shellfire, rockets, airstrikes and barrel bombs pounding the area and crippling food shortages.
Terrified residents have been seeking refuge in basements, but many have not been able to escape and footage has shown adults and children pulled out of rubble after attacks.
The Syrian government insists that it is fighting terrorists who are also attacking Damascus - and that it does not intentionally attack civilians.
A UN vote on a 30-day ceasefire in the whole of Syria was due to take place, after Sweden and Kuwait tabled a proposed resolution.
It would allow food, aid and medical evacuations from eastern Ghouta and other areas.
However Russia has said there was no agreement and has presented amendments to the resolution.
The country’s ambassador said Sweden and Kuwait, which drafted the measure, had requested a vote on the draft resolution even though they are "fully aware there is no agreement on it."
UN Syrian envoy Staffan de Mistura said he hoped the resolution would be passed but admitted it was an "uphill" struggle.
Russian foreign minister Sergey Lavrov has said the Kremlin will consider supporting the UN ceasefire resolution if it does not cover fighters from IS and the al Qaeda-linked Levant Liberation Committee.
Moscow retains the power to veto any resolution.
Reporting from IRN