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.
At least 27 people have been killed in the strikes, monitors say, although other reports put the death toll higher. 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, says dozens of air strikes were carried out in Aleppo on Friday morning.
The group says the centres set up to help victims of bombardments were being targeted, and three out of four had been put out of action. One White Helmet volunteer reported air strikes as he carried out an interview with the BBC World Service.
The Syrian Observatory for Human Rights monitoring group said at least 27 civilians, including three children, were killed and dozens more wounded in the latest air strikes.
Ammar al-Selmo, the head of civil defence in eastern Aleppo, told Reuters news agency that the death toll was more than 70, with at least 40 buildings destroyed.
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.
Bitterness on all sides - BBC diplomatic correspondent Jonathan Marcus
Russian and Syrian commanders - always sceptical about the merits of the ceasefire - now appear to have been given a green light to sort out the Aleppo problem once and for all.
Imagery of the shattered city presents a gaunt vista in which thousands of innocent people remain trapped on both sides. Targeted attacks by government forces against medical and civil defence facilities add another element of horror to the situation.
The fact that this new onslaught is taking place against a backdrop of the total failure in the US and Russian brokered efforts to implement a ceasefire in Syria is also significant. In the wake of this failure there is a good degree of bitterness on all sides.
This provides space for the stepping up of military action on the ground. Indeed the pause may well have enabled government forces to reposition in key areas and to improve intelligence gathering.
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 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".
Mr Kerry met Mr Lavrov again on Friday and said there had been "a little bit of progress" on resolving differences.
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."