Four air strikes have hit rebel-held parts of the Syrian city of Aleppo, activists say, the first raids there since a ceasefire began last Monday.
Several people were injured, the UK-based Syrian Observatory for Human Rights said, but it did not say who carried out the strikes.
Russia has said the future of the truce is in doubt after the US-led coalition struck the Syrian army in Deir al-Zour.
A Syrian political adviser told the BBC the US strike could not be a mistake.
The cessation of hostilities brokered jointly by the US and Russia does not exclude attacks on IS or other jihadist groups.
An activist in Aleppo confirmed there had been strikes on the eastern neighbourhoods of Karam al-Jabal and al-Shaar.
The Aleppo Media Centre reported three injuries in an air strike on al-Sakhour neighbourhood.
The US attack on Saturday has led to a deterioration in relations between the US and Russia, with both Russia and Syria saying it proved there was co-ordination between the US and the Islamic State group.
The US military said the coalition believed it was attacking IS positions, and has expressed regret for the "unintentional loss of life".
But President Bashar al-Assad's media adviser Bouthaina Shaaban said she did not believe the attack was unintentional.
"The United States, the superpower, the greatest country in the world, makes mistakes in targeting the army? I mean, this doesn't make sense to ask," she told BBC World television.
"The other explanation is that there is one authority in the United States who wanted to conduct this, the other doesn't want to. And that's why they are finding it very difficult to implement what they agreed upon with the Russian," she said.
The attack put "a very big question mark" over the truce's future, said Russia's UN Ambassador Vitaly Churkin.
But French Foreign Minister Jean-Marc Ayrault said that despite the US-led strike the Syrian government was mostly to blame for the violations of the truce.
Syria said on Sunday it had lost a warplane in the area of Saturday's attack, which so-called Islamic State (IS) said it had shot down.
The BBC's Lina Sinjab, reporting from neighbouring Lebanon, says it is rare for the Syrian government to acknowledge losses caused by IS, and the announcement may be intended to draw attention to the consequences of the US-led air strike.
The air attack caused a bitter row between the US and Russia at the United Nations Security Council on Saturday night, with each country's representatives walking out while the other was speaking.