The truce in Syria has been violated nine times in the past 24 hours, says Russia, but is otherwise holding.
A temporary cessation of hostilities was agreed as part of a US-Russian plan and is now in its second day.
Reports say warplanes have attacked villages to the west of Aleppo. It it is unclear whether the planes were Russian or Syrian.
The truce is the first major cessation of hostilities in the five years of war which have killed more than 250,000.
Millions more have been forced from their homes.
Humanitarian agencies are hoping to deliver aid to besieged areas of the country.
Some activists say the target of the air strikes 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 ceasefire breach.
But the villages also contain fighters from the Western-backed opposition as territorial control in Syria is often blurred, says the BBC’s Mark Lowen on the Turkish border,
Meanwhile Russia says it has detected a cross-border attack from Turkey near Tal Abyad and 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 strikes near Tal Abyad.
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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 Syrian opposition umbrella group the High Negotiations Committee (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.