U.S. officials are quoted as saying four of the cruise missiles that Russia has fired at Syrian targets crashed in Iran.
The unidentified officials said it was unclear whether the missiles caused any damage and provided no details of where they might have landed.
The Russian Defense Ministry denied the claim.
Moscow announced on October 7 it had launched 26 cruise missiles from a Russian ship in the Caspian Sea, hitting targets in northern and northwestern Syria.
Earlier on October 8, NATO Secretary-General Jens Stoltenberg warned of a "troubling escalation" of Russian military activity in Syria, as Damascus announced a wide-ranging ground offensive backed by more Russian air strikes.
The strikes have "weakened" the Islamic State (IS) militant group, Syrian Army Chief of Staff Lieutenant General Ali Abdullah Ayoub said, enabling the army to start a "big attack" to retake towns and villages.
Moscow denies Western accusations it has mainly targeted opponents of Syrian President Bashar al-Assad, saying its strikes have hit positions of the IS and other extremist groups.
