"At this time, we have no information to corroborate press reports that the aircraft was shot down," a U.S. defense official said, speaking on condition of anonymity.

"We are looking into the incident and will provide more details when available."

Another U.S. official, speaking to Reuters on condition of anonymity, said flatly the United States could not "determine the cause yet," without elaborating.

It was the first such incident since the U.S.-led coalition began carrying out air strikes against the hardline militant Islamic State group in Iraq and Syria last summer.

U.S. officials have previously described Syrian skies as relatively passive, noting that the Syrian government of President Bashar al-Assad had not taken aim at coalition aircraft flying over its territory.