A US military spokesman in Baghdad said Russians notified the US-led coalition shortly before their warplanes took off from Iran on Tuesday to bomb Islamic State militants in Syria.
Speaking to reporters at the Pentagon via a video link, Colonel Christopher Garver said the Russian bomber flights did not affect US coalition air operations.
The overflight could potentially put Russian planes in proximity of US and coalition aircraft in Iraq and Syria.
According to Syrian opposition monitoring groups, an airstrike on a city in eastern Syria - in a neighborhood held by the ISIS group - has left ten people dead.
They also say a wave of airstrikes on rebel-held parts of the northern city of Aleppo have killed at least 15 civilians and wounded many others.