Syrian or Russian warplanes bombed the city of Aleppo and nearby villages on Monday, the Syrian Observatory for Human Rights reported, after the Syrian military ended a seven-day ceasefire.
The airstrikes hit rebel-held areas in central Aleppo and villages to the west of the city, the British-based monitoring group said, reporting a number of dead and injured.
Earlier, Syria's military declared the initial period for a cease-fire brokered by the United States and Russia over, even as officials from the United States and Russia met behind closed doors in Geneva to try to extend it.
In the Syrian army statement, it said that "terrorist groups", a term the Syrian government uses to refer to all insurgents fighting against it, had failed to commit to any of the provisions of the deal, and had used the cease-fire to rearm whilst violating it 300 times.
It vowed to "continue fulfilling its national duties in fighting terrorism in order to bring back security and stability".
U.S. Secretary of State John Kerry said it was too early to call the cease-fire finished, and the United Nations said that only Washington and Moscow could declare it over, as they were the ones who had originally agreed it.
"It would be good if they didn't talk first to the press but if they talked to the people who are actually negotiating this," Kerry said. "We just began today to see real movement of humanitarian goods, and let's see where we are. We're happy to have a conversation with them."
Aid was delivered to the besieged town of Talbiseh in Homs province on Monday, the Red Cross said, for the first time since July. The convoy brought in food, water and hygiene supplies for up to 84,000 people, it said.
But most aid shipments envisioned under the truce have yet to go in, especially a convoy destined for rebel-held eastern parts of Aleppo, where some 275,000 civilians are believed trapped without access to food or medical supplies.
"I am pained and disappointed that a United Nations convoy has yet to cross into Syria from Turkey, and safely reach eastern Aleppo," the UN Under Secretary General for Humanitarian Affairs Stephen O'Brien said in a statement.
