Warplanes have launched fresh air strikes in Aleppo, hours after Syria's government announced an offensive to retake rebel-held areas of the city, reports BBC.
Syrian and Russian planes were pounding eastern Aleppo, according to activists. A rescue worker described what was happening as "annihilation".
The government has urged people to avoid positions held by rebels. Russia has not confirmed its involvement.
US-Russian talks to revive a collapsed truce have broken up without progress.
Russia supports the Syrian government, while the US backs the opposition. The two world powers accuse each other of failing to rein in their respective allies on the ground.
The White Helmets, a Syrian volunteer rescue group, says dozens of air strikes were carried out in Aleppo on Friday morning.
It say the centres set up to help victims of bombardments were being targeted, and three out of four had been put out of action.
Announcing the new offensive on state television late on Thursday, the government warned Aleppo residents to "stay away" from "terrorist positions".
It was unclear whether the new offensive would involve ground troops. But army officials said there would be exit points for anyone, including rebels, who wanted to flee.
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 for Syria Staffan de Mistura described them as "painful and disappointing".
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".
Also on Thursday, Syrian President Bashar al-Assad 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."