Anti-government rebels and Russia have both reported breaches of the fragile truce in Syria – now in its second day.
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.
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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.
UN special envoy for Syria Staffan de Mistura has said that peace talks will resume on 7 March if the truce “largely holds”, adding that he had no doubt there would be “no shortage of attempts to undermine this process”.
The cessation was brokered by the US and Russia, and is backed by a UN resolution. Previous talks in Geneva collapsed in early February after making no progress.
The UN resolution names about 30 areas in dire need of aid, including eastern and western rural Aleppo and the eastern city of Deir al-Zour, which is under siege by IS.
Almost 100 rebel factions have agreed to respect the truce, the HNC says.
However, the HNC warned the Syrian government and its allies not to use the “proposed text to continue the hostile operations against the opposition factions under the excuse of fighting terrorism”.
Russian President Vladimir Putin says his forces are targeting IS, Nusra Front and other extremist groups designated as legitimate targets by the UN Security Council.
However, over the course of its campaign in Syria, Russia has been widely accused of also attacking more moderate rebel groups fighting President Bashar al-Assad, an ally of the Kremlin.
Syria’s civil war
Why is there a war in Syria?
Anti-government protests developed into a civil war that four years on has ground to a stalemate, with the Assad government, Islamic State, an array of Syrian rebels and Kurdish fighters all holding territory.
What’s the human cost?
More than 250,000 Syrians have been killed and a million injured. Some 11 million others have been forced from their homes, of whom four million have fled abroad – including growing numbers who are making the dangerous journey to Europe.
How has the world reacted?
Iran, Russia and Lebanon’s Hezbollah movement are propping up the Alawite-led Assad government, while Turkey, Saudi Arabia and Qatar back the more moderate Sunni-dominated opposition, along with the US, UK and France. Hezbollah and Iran have pro-Assad forces on the ground, while a Western-led coalition and Russia are carrying out air strikes.