Warplanes have launched fresh air strikes in Aleppo, hours after Syria's government announced an offensive to retake rebel-held areas of the city.
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.
The 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.