Russian planes have attacked several sites in northern Syria, activists say.
Russia has not confirmed any sorties on Sunday and says it has also identified nine breaches of the truce.
A cessation of hostilities was agreed as part of a US-Russian plan. Russia says that in general it is holding. It is the first major cessation of hostilities in the five years of war.
More than 250,000 have been killed in the fight against President Bashar al-Assad.
Millions more have been forced from their homes. Humanitarian agencies are hoping use the truce to deliver aid to besieged areas of the country.
- What's the difference between 'ceasefire' and 'cessation'?
- Syria truce: Who's in and who's out?
- Syria conflict at 'hinge' moment
- How Putin is succeeding in Syria
- Tide turns Assad's way
The Syrian opposition has complained of 15 violations of the ceasefire by the government side, which is supported by Russia.
The Syrian opposition umbrella group the High Negotiations Committee (HNC) said it would be sending a formal letter of complaint about the breaches to the UN and other world powers.
However, the HNC said that despite violations "here and there", it was "positive to see people getting relief …to be safe, and free from fear".
Some Syrian activists say the target of the air strikes in the north, near Aleppo, was the al-Qaeda-linked Nusra Front.
The truce involves Syrian government and rebel forces, but not the so-called Islamic State group (IS) or the Nusra Front, so an attack on them would not count as a breach.
But the villages also contain fighters from the Western-backed opposition as territorial control in Syria is often blurred, reports the BBC's Mark Lowen from the Turkish border.
Given the terms of this ceasefire deal, it is conceivable that the rebels were targeted under cover of striking the Nusra Front, he adds.
The HNC said two of the strikes were in areas where designated terrorist groups were not operating.
Meanwhile, among the breaches reported by the Russian military was what it described as a "cross-border" attack from Turkey near Tal Abyad. Russia has asked the US to investigate.
The US military told the BBC that it had continued to attack IS targets in Syria on Saturday, including 10 air strikes near Tal Abyad.
The "cessation of hostilities" began at midnight on Saturday (22:00 GMT Friday).
The Russian Foreign Minister, Sergei Lavrov and the US Secretary of State John Kerry spoke by phone on Saturday welcoming the ceasefire and, Moscow says, discussing ways of supporting it through military co-operation.