Aid trucks have been hit by airstrikes near Aleppo, the Syrian Observatory for Human Rights has reported.
Syria's military has declared a week-long ceasefire over, with air raids reported in Aleppo as US and Russian officials met in Geneva to try to extend the truce.
Warplanes bombed rebel-held areas in the city of Aleppo and nearby villages, the Syrian Observatory for Human Rights said on Monday, reporting a number of dead and injured.
The monitoring group said it was not clear if the jets were Syrian or Russian. Moscow supports President Bashar al-Assad with its air force.
The raids came as what is likely to be the final attempt by the US administration of President Barack Obama to find a negotiated solution to the five year old civil war appeared close to collapse.
US Secretary of State John Kerry said it was too early to call the ceasefire finished and the United Nations said only Washington and Moscow could declare it over, as they were the ones who originally agreed it.
Washington said it was working to extend the truce but called on Russia to first clarify the Syrian army's statement that it was over.
Russian and US officials met in Geneva on Monday and the International Syria Support Group - the countries backing the Syria peace process - were scheduled to meet on Tuesday in New York to assess the ceasefire agreement.
But both the Syrian army and the rebels spoke openly of returning to the battlefield.
Syria's army said the seven-day truce period had ended.
It accused "terrorist groups", a term the government uses for all insurgents, of exploiting the calm to rearm while violating the ceasefire 300 times and vowed to "continue fulfilling its national duties in fighting terrorism in order to bring back security and stability".
Asked about the army's statement, Kerry told reporters in New York the seven days of calm and aid deliveries envisaged in the truce had not yet taken place.
"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.
Already widely violated since it took effect, the ceasefire came under added strain at the weekend when Russia said jets from the US-led coalition against Islamic State killed more than 60 Syrian soldiers in eastern Syria.
Assad called that incident "flagrant aggression". Washington has called it a mistake.