War planes have bombed six towns in Syria's northern Aleppo province, a monitoring group and insurgents have confirmed.
The airstrikes come a day after a cessation of hostilities came into effect.
Syrian insurgents claim the strikes were carried out by Russian war planes.
However the UK-based Syrian Observatory for Human Rights said the identity of the jets is not yet clear.
The break in fighting, brokered by the US and Russia, began at midnight local time on Saturday, (2200 GMT on Friday).
It had been described as Syria's best hope for peace in the five-year civil war that has claimed 270,000 lives and displaced more than half the population.
The temporary truce, which does not apply to so-called Islamic State and the al Qaeda-linked al Nusra Front, largely held throughout Saturday despite some reports of violations on both sides of the conflict.
Russia, which has been propping up the Syrian government with airstrikes, had said it would continue its bombing campaign against terrorist organisations throughout the cessation in hostilities.
Moscow did, however, pledge to halt strikes for the day on Saturday to ensure no wrong targets were hit by mistake.
As Saturday drew to a close, an international task force set up to monitor the fighting, co-chaired by the United States and Russia, said the first day of the truce had been largely successful.
"The United Nations, the United States and Russia have made a positive assessment of the first hours of the cessation of hostilities," a Western diplomat said after a meeting of the International Syria Support Group in Geneva.
"Some incidents" in apparent violation of the truce had been "defused", he added.
Among the potential breaches were reports in Syrian state media of shelling in Syria's capital Damascus. No casualties were reported.
Rebels also accused government forces of intermittent "truce violations".
Islamic State militants, meanwhile, kept up their offensive, storming a border town in Raqa province, sparking clashes that killed at least 70 IS members, 20 Kurdish militiamen and two civilians, the Observatory said.
US-led coalition warplanes launched at least 10 air strikes to repel the assault, it reported.
Twin suicide bombings also killed six people in Hama province, where Islamic State is present.
