Four Russian cruise missiles fired at Syria from the Caspian Sea landed on Iran, unnamed US officials say.
It was unclear whether the missiles caused any damage, they said.
Russia’s defence ministry has declined to comment. On Wednesday, Russia said it had launched 26 cruise missiles at targets in north and north-west Syria.
The news came as Nato renewed assurances to defend its allies in view of the “escalation of Russian military activities” in Syria.
Nato is boosting its response forces to be able to deploy troops speedily.
Moscow denies Western accusations that it has mainly targeted opponents of Syrian President Bashar al-Assad, insisting its strikes have hit the infrastructure of the so-called Islamic State (IS) and other militant groups.