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.