Fighter jets pounded rebel-held areas of Aleppo with dozens of airstrikes early Friday, local activists and rescue groups said, as prospects of reviving a cease-fire looked increasingly grim.
The Syria Civil Defense service — known as the White Helmets — said three of its four facilities in the east of Aleppo were "deliberately targeted" in a wave of raids that started at 6 a.m. local time (11 p.m. Thursday ET).
"What's happening now is annihilation," Ammar al Selmo, the head of the service in Aleppo, told Reuters.
The U.K.-based Syrian Observatory for Human Rights, meanwhile, put the death toll at 15 but said it expected the number to rise. Conflicting death tolls are common in the immediate aftermath of strikes.
Friday's raids in Aleppo, a city of 250,000, came just hours after Secretary of State John Kerry met with members of the White Helmets on the sidelines of the United Nations General Assembly.
A U.S.-Russia brokered cease-fire had lasted around a week but unraveled following a deadly strike on a United Nations aid convoy and resumption in violence.
Syria's military on Thursday said it would restart bombing raids in the area, urging civilians to "stay away" from locations and facilities tied to rebel groups.
The new offensive coincided with a testy meeting about Syria in New York between key players — including Kerry and Russian Foreign Minister Sergey Lavrov — ended without an agreement.
A senior administration official described the two-and-a-half hour meeting as "pretty contentious" and said it was hard to see a return to the cease-fire while Syria's government conducts its raids in eastern Aleppo.
"The ball is in very much in the Russians' court to come back to us with an idea or some ideas that are serious," the senior official told reporters.
Reuters reported that Kerry was informed of the Syrian government offensive when his chief of staff showed him a headline on his BlackBerry.
A furious Kerry then told the entire room, Lavrov included, that "even while we are meeting here, they are doing this," an official told Reuters on condition of anonymity.
Lavrov had sought a three-day pause in fighting to revive the cease-fire. But U.S. officials said there was no point returning to a situation in which rebels would be pressed to hold fire while the Syrian and Russian military could violate the agreement.
A medic with the Syrian aid group Shafak was seriously hurt in an attack as Thursday's government offensive began, according to the group's partner, Save the Children.
It said the 25-year-old sustained injuries to his chest and eyes, as well as burns on 20 percent of his body, when a phosphorous bomb hit his building when he was leaving to go to work. He was hit from shrapnel from a bomb as he tried to put out the fire sparked by the strike, the charity added.
"Despite the international outcry over the bombing of a UN convoy earlier this week, once again another Syrian humanitarian has been hurt," Nick Finney, North-West Syria Country Director for Save the Children, said in a news release.
Syria's President Bashar al-Assad has blamed the U.S. for the cease-fire deal's failure, citing what he called America's inability to control "terrorist" groups and a weekend attack that killed dozens of Syrian soldiers.