Warplanes have continued to pound the northern city of Aleppo, as Syria's military presses ahead with its new offensive to retake rebel-held areas.
Aleppo has endured a day of devastating air strikes with at least 45 civilians killed, the Syrian Observatory for Human Rights monitoring body said.
But the head of a hospital in the rebel-held east told Reuters that 91 people had been killed in Friday's bombardment.
One rescue worker described what happened as "annihilation".
The government has urged people to avoid positions held by rebels.
US-Russian talks at the UN have failed to revive a collapsed truce.
Activists said both Syrian and Russian warplanes were taking part in the offensive, though Russia has not confirmed its involvement.
Russia supports the Syrian government, while the US backs the opposition. The two powers accuse each other of failing to rein in their respective allies on the ground.
The White Helmets, a Syrian volunteer rescue group, said dozens of air strikes were carried out in Aleppo on Friday morning.
In between the raids, White Helmet volunteers frantically searched for those trapped in the rubble of demolished buildings, often with their bare hands.
The group said centres set up to help victims of bombardments were being targetted, and three out of four had been put out of action.
Unicef said that nearly two million people were again without running water after an attack on a pumping station that served the rebel-held east of the city, and the retaliatory shutdown of a station serving the government-controlled west.
The Al Jazeera news agency tweeted that its bureau in the city had been partly destroyed.
Announcing the new offensive on state television late on Thursday, the Syrian government warned Aleppo residents to "stay away" from "terrorist positions".
Syrian military sources said a ground offensive would follow.
One told the Agence France-Presse news agency that the bombardment "could go on for hours or days before the ground operation starts. The timing of the ground operation will depend on the results of the strikes".
Army officials said there would be exit points for anyone, including rebels, who wanted to flee.
The government's warning came after days of air strikes on Aleppo. Syria declared the week-long ceasefire over on Monday.
Residents told the BBC earlier this week that barrel bombs had struck rebel-held districts, causing many fires. Dozens of civilians were reported to have been killed.
Thursday night's meeting in New York brought together members of the International Syria Support Group, which includes the US, Russia and other powers.
After the talks broke down without agreement, UN envoy Syria Staffan de Mistura described them as "painful and disappointing".
US and Russia meet
US Secretary of State John Kerry said Washington could not be the only one trying to hold open the door to peace.
Russian Foreign Minister Sergei Lavrov accused US-backed rebels of ignoring the truce, and said a new one would amount to a "unilateral pause".
Mr Kerry met Mr Lavrov again on Friday and said there had been "a little bit of progress" on resolving differences.
Mr Lavrov told the UN that the US and Russian plans to end Syria's conflict must be saved as there was no alternative.
Mr Lavrov laid the blame on the US for failing to control the rebel groups it backs.
He said a key condition of the truce was for moderate rebel groups backed by the US to separate themselves from militants.
"Unfortunately the coalition led by the United States, which committed itself to make sure that this separation happens, has not been able to do this," Mr Lavrov said, although he said his "good friend" Secretary of State John Kerry had indicated this remained the commitment of the United States.
Mr Lavrov said that if the location of militants of the Nusra Front could be pinpointed, he remained convinced a cessation of hostilities and a delivery of humanitarian aid would be possible.
He said it was "now essential to prevent a disruption" of the US-Russia agreements.
Syrian President Bashar al-Assad on Thursday denied claims that he was responsible for the ongoing fighting.
In an interview with the Associated Press news agency, he reaffirmed his government's position that a deadly strike on an aid convoy in Aleppo on Monday had not been carried out by Syrian or Russian planes.
Mr Assad also ridiculed concern about the army's use of barrel bombs, saying: "What's the difference between different kinds of bombs? All bombs are to kill, but it's about how to use it. When you use armament... you kill terrorists in order to defend civilians."