Russia's defence ministry denied a claim by a US official Thursday 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.

The US official, who spoke on condition of anonymity, said the missiles landed in Iran on Wednesday, but did not provide details about where they might have landed or whether they caused any damage.