BRUSSELS — Four cruise missiles in a barrage of 26 fired by Russia’s warships in the Caspian Sea at rebel targets in Syria crashed in a rural area of northern Iran instead, senior United States officials said on Thursday.

Russian and Iranian officials dismissed the claim as a nonsensical propaganda ploy, as the Kremlin intensified military coordination with the newly emboldened forces of President Bashar al-Assad of Syria to turn the tide of the war. Russian warplanes flew supporting missions for Syrian troops advancing on insurgent positions in a hotly contested area of Hama Province, rebels and a monitoring group in Syria reported.

It was unclear exactly where in Iran the Russian missiles might have landed, or whether they caused any casualties or damage. The United States officials said the flight path of the Russian cruise missiles, called Kalibrs, would have taken them across northern sections of Iran and Iraq en route to Syria.

Of the 26-missile volley, the officials said, four went awry and hit northern Iran, according to technical sources of information like radar and satellite imagery.