Syria's opposition and civil society members are criticising what they viewed as Russian complicity in the bombardment of Aleppo, which has killed hope of reviving a ceasefire.
Warplanes bombed Aleppo on Friday (local time) with what residents described as unprecedented ferocity after the Syrian army declared an offensive to fully capture Syria's biggest city.
The number of people killed in the bombardment of rebel-held eastern districts of the Syrian city of Aleppo on Friday alone is 91, says Hamza al-Khatib, the director of a hospital there.
The war pits Syrian dictator Bashar al-Assad, backed by Russia, Iran and Arab Shi'ite militias, against Sunni rebel groups, including some supported by Washington, Turkey and Gulf Arab states.
Asked about the Aleppo bombardment, Bassma Kodmani, a member of the opposition High Negotiations Committee, said it was a "demonstration of the implications of the failure of yesterday's meeting".
The United States and Russia failed on Thursday (local time) at the United Nations to agree on how to revive a short-lived ceasefire.
"There's every indication that they (Russia) entirely condone the massive attack now on Aleppo and ... not only support it, they are part of it," Kodmani said.
US President Barack Obama has been deeply reluctant to use more military force in Syria, a policy that has been the source of exasperation among some European and Gulf allies.
Kodmani said only a "credible threat of retaliation" would stop the Assad regime's warplanes.
Russia and the United States on September 9 agreed to a deal aimed at putting Syria's peace process back on track.
It included a nationwide truce, improved humanitarian aid access and the possibility of joint military operations against al-Qaeda-lined Nusra Front and Islamic State.
The truce effectively collapsed after a week when an aid convoy was bombed on Monday, killing some 20 people.
US Secretary of State John Kerry, who has tried to revive a deal with Russia, met with representatives of Syrian civil society groups on Thursday (local time). They said the indiscriminate bombing of civilians must stop.
"Kerry and Obama's administration are going in circles ... talking to the Russians, thinking that the Russians will have any solutions and can contribute to the ceasefire. It's clear that the Russians will not do this," Mutasem Alsyofi of the Syrian Civil Society Declaration Initiative told Reuters in New York.