The official, who spoke on condition of anonymity, said the missiles landed in Iran yesterday, but did not provide details about where they might have landed or whether they caused any damage.
Nor would the official say exactly what type of cruise missiles went down, except to say that they were among a salvo fired Wednesday from Russian warships in the Caspian.
The Russian Defence Ministry posted a graphic on its website showing 26 missiles overflying Iran and Iraq before striking inside Syria.
The missile launches were in support of a major ground offensive by the Syrian army on several fronts in the war-torn country's west.
US Defence Secretary Ashton Carter today deplored the fact that Russian warships fired the cruise missiles "without warning.
