Four Russian cruise missiles aimed at targets in Syria instead crashed in Iran, a US official said Thursday, declining to comment on whether there were any casualties.
The 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.
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 Defense 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.
While Russia has been claiming it is targeting Islamic State (ISIS), the US 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 US-backed "moderate" rebel groups fighting ISIS.
US Defense Secretary Ashton Carter on Thursday deplored the fact that Russian warships fired the cruise missiles "without warning."
Carter added that Russia will soon begin to suffer casualties after dramatically expanding its military role in Syria to prop up President Bashar al-Assad.
"This will have consequences for Russia itself which is rightly fearful of attacks...in coming days, the Russians will begin to suffer from casualties," Carter said at a NATO defense ministers meeting in Brussels dominated by the Syrian crisis.
AFP contributed to this report.