Update at 11pm: A Syrian activist monitoring group says that 12 aid workers and truck drivers were killed when their humanitarian aid convoy was hit by airstrikes in Aleppo province.
The Britain-based Syrian Observatory for Human Rights, which relies on a network of activists inside the country, reported the casualty figures on Monday.
The report could not be independently verified.
Jan Egeland, humanitarian aid coordinator in the office of the UN envoy for Syria, said that the Syrian Red Crescent convoy carrying UN supplies had been "bombarded".
Mr Egeland added: "It is outrageous that it was hit while offloading at warehouses."
Update at 9.45pm: The United Nations has confirmed air strikes have hit trucks in an aid convoy near the Syrian city of Aleppo.
The attack came just hours after the government there declared a ceasefire was over.
The Red Cross says the bombing killed more than 10 people.
Update at 9.20pm: The US has said it is prepared to extend the window for Syria's fractured week-old ceasefire despite numerous violations and the Syrian military's announcement that the truce is over.
The State Department said it was ready to work with Russia to strengthen the terms of the agreement and expand deliveries of humanitarian aid but added that Russia must clarify its position on the status of the ceasefire.
Spokesman John Kirby noted the Syrian announcement while stressing that the US and Russia agreed to the arrangement, but the government of Syrian president Bashar Assad has not.
"We are prepared to extend the cessation of hostilities, while working to strengthen it and expand deliveries of assistance," Mr Kirby said in a statement released after the Syrian declaration.
"We will be consulting with our Russian counterparts to continue to urge them to use their influence on Assad to these ends.
"While we have seen comments attributed to the Syrian military, our arrangement is with Russia, which is responsible for the Syrian regime's compliance, so we expect Russia to clarify their position."
Russia's Foreign Ministry said the failure of Syrian rebels to adhere to the truce "threatens the cease-fire and US-Russian agreements".
The ministry statement came after the Russian military said that continuing rebel violations made it "meaningless" for the Syrian army to respect the deal.
The Syrian military said earlier on Monday that the ceasefire had expired.
While acknowledging numerous violations, Mr Kirby said the truce, which took effect last Monday, had been responsible for "a measure of reduced violence".
However, he also repeated calls for the sustained delivery of humanitarian aid to Aleppo and other besieged communities.
Such deliveries began only on Monday and were available only in limited areas, he said.
Earlier, Secretary of State John Kerry expressed hope that the ceasefire could still hold even after the Syrian military's announcement and took aim at Russia for not doing enough to pressure Mr Assad's government to comply.
"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," he told reporters on the sidelines of the UN General Assembly.
"As I said yesterday, (it's) time to end the grandstanding and time to do the real work of delivering on the humanitarian goods that are necessary for access."
Mr Kerry expressed frustration with the touch-and-go ceasefire.
"We have not had seven days of calm and of delivery of humanitarian goods," Mr Kerry said.
Those seven days of calm and aid deliveries were required before the US and Russia could embark on a plan to co-operate in targeting the Islamic State group and al Qaida affiliates working in Syria.
The Syrian military said in a statement on Monday that "armed terrorist groups" repeatedly violated the ceasefire and took advantage of the truce to mobilise and arm themselves while attacking government-held areas.
The statement said the rebels wasted a "real chance" to stop the bloodshed.
He said US and Russian officials were meeting in Geneva to try to sort out aid deliveries to Aleppo and other besieged communities.
American officials said, however, that conditions were still not right for US-Russian military co-operation.
A Syrian activist group said 92 people have been killed in Syria since the start of the ceasefire.
The Britain-based Syrian Observatory for Human Rights said 29 children and teenagers were among those killed, as well as 17 women.
The figure does not include dozens of Syrian soldiers and Islamic State militants killed in the eastern province of Deir el-Zour, the Observatory said on Monday.
A mistaken air raid by the US-led coalition also killed 62 Syrian soldiers.
The opposition reported 254 violations by government forces and their allies since the truce started on September 12 and a senior Syrian opposition official declared the ceasefire "clinically dead".
Syrian state media said there were 32 violations by rebels on Sunday alone.