Russia's defense ministry denies American claim that Syria-bound Russian cruise missiles fired from the Caspian Sea had crashed in Iran.

Russia's defense ministry on Thursday denied a claim by an American official that four Syria-bound Russian cruise missiles fired from the Caspian Sea had crashed in Iran.

"Any professional knows that during these operations we always fix the target before and after impact. All our cruise missiles hit their target," ministry spokesman General Igor Konashenkov said in a statement quoted by the AFP news agency.

The American official, who spoke on condition of anonymity, said the missiles landed in Iran on Wednesday, confirming a story first reported by CNN.

But he 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 from Russian warships in the Caspian.

"In contrast to CNN we do not refer to anonymous sources but we show the path of our missiles and their targets practically in real time," Konashenkov was quoted as having said.

The Russian defense ministry posted a graphic on its website on Wednesday showing 26 missiles overflying Iran and Iraq before striking inside Syria.

Russia conducted its first airstrikes in Syria last week, and on Wednesday Moscow ramped up its war by unleashing cruise missiles from the Caspian Sea.

While Russia has been claiming it is targeting Islamic State (ISIS), the State Department said Wednesday that most of Russia's barrage of airstrikes in Syria are not hitting ISIS terrorists as it claims.

In the first Russian strikes against the Homs and Hama districts of the country, an unnamed American official later said the targets were "moderate" rebel groups fighting ISIS.