Syrian government air raids killed a further 32 people in rebel-held suburbs east of Damascus on Friday, activists said, as a vote on a UN-backed ceasefire was delayed.
UN Security Council members had been discussing a cessation of hostilities for 30 days, to allow aid to be brought in and the injured to be taken out.
But negotiations were held up over a number of issues, including when the ceasefire should begin.
The US blamed Russia, which proposed an amendment to delay it.
US Ambassador to the UN Nikki Haley expressed dismay, saying on Twitter: “Unbelievable that Russia is stalling a vote on a ceasefire allowing humanitarian access in Syria.
“How many more people will die before the Security Council agrees to take up this vote? Let’s do this tonight. The Syrian people can’t wait.”
The towns and farms of eastern Ghouta, a suburb to the east of the capital Damascus, have been under government siege since 2013.
On Friday, the densely populated enclave was bombed for a sixth straight day, witnesses said.
At least 462 people have been killed since the bombing intensified last week, including at least 99 children, the Syrian Observatory for Human Rights monitoring group said.
Russia’s UN Ambassador Vassily Nebenzia called an immediate ceasefire unrealistic.
The Russian amendment was rejected by Sweden and Kuwait, sponsors of the proposed resolution.
The ambassador of Kuwait, which holds the council presidency, said agreement was close, but there was still no consensus on the draft resolution.
One of the stumbling blocks was a Russian demand that rebel groups fighting President Bashar al Assad’s forces comply with the truce.
In a concession to Russia, the latest draft of the resolution states that operations against Islamic State, al Qaeda, and “individuals, groups…” associated with the extremist groups will not be included.
It also proposes the immediate lifting of all sieges, including in eastern Ghouta, Yarmouk, Foua and Kefraya and ordering all sides to “cease depriving civilians of food and medicine indispensable to their survival”.
The initial resolution called for the ceasefire to begin 72 hours after its adoption, but diplomats continued discussions into the night about the final wording.
Swedish Ambassador Olof Skoog told reporters on Friday night: “We have not been able to close the gap completely.
“We will work tonight and hopefully, we will definitely come back tomorrow and we will vote.”
The proposal was for a vote at midday EST (5pm UK time).
Mr Skoog said earlier: “It is about saving lives.
“UN convoys and evacuation teams are ready to go. It’s time for the council to come together and shoulder its responsibility to urgently avert a situation that is beyond words in its desperation.”
On Thursday, Syrian army helicopters dropped leaflets over eastern Ghouta, urging residents to leave for their own safety and calling on opposition fighters to surrender.
The 32 killed on Friday died in raids on areas including Hammouriyeh, Zamalka, Douma and al Marj, according to the British-based Syrian Observatory for Human Rights.
The Ghouta Media Centre, run by activists, also reported 32 killed.
Syrian state TV reported that militants fired 70 shells at Damascus, with one person killed and 60 others wounded.
Russia has so far vetoed 11 draft resolutions on Syria that targeted its ally in Damascus.