Four Russian cruise missiles fired at Syria from the Caspian Sea landed in Iran, unnamed US officials say.
It was unclear whether the missiles caused any damage, they said.
On Wednesday, Russia said it had fired 26 missiles at 11 targets in north and north-west Syria - about 1,500km away. On Thursday, it reiterated that all the rockets hit their targets.
The news came as Nato renewed assurances to defend its allies in view of Russia's "escalation" in Syria.
Russia's ambassador to Ankara had earlier been summoned three times in response to recent violations of Turkey's airspace. Turkey is a Nato member.
The military alliance is also boosting its response forces to be able to deploy troops speedily.