Russian defence officials say their aircraft carried out about 20 missions against the so-called Islamic State group (IS) on Wednesday.
But the US expressed fears the targets were non-IS opponents of Russia's ally, Syrian leader Bashar al-Assad.
The US is targeting IS with air strikes in both Syria and Iraq.
Nato said there had been little co-ordination by Russia with US-led forces against IS, also known as Isil.
The US says it was informed of Wednesday's air strikes only an hour before they took place.
What if the targets weren't ISIS and turn out to be kalab Bashar's allies?
But the US expressed fears the targets were non-IS opponents of Russia's ally, Syrian leader Bashar al-Assad.
The US is targeting IS with air strikes in both Syria and Iraq.
Nato said there had been little co-ordination by Russia with US-led forces against IS, also known as Isil.
The US says it was informed of Wednesday's air strikes only an hour before they took place.
What if the targets weren't ISIS and turn out to be kalab Bashar's allies?

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