Asked about the assurance that Syrian government forces would not be targeted, a senior U.S. State Department official told Reuters: “We communicated our intentions, but not specific timing or targets, to the Iranians. As we’ve said, we won’t be coordinating military action with Iran. And of course we won’t be sharing intelligence with Iran either.”

Assad is the closest regional ally of Iran, which has provided military support to his government during its civil war, now in its fourth year.

Tehran has called on the world to fight the militants of Islamic State, which stands accused of a wave of sectarian violence, beheadings and massacres of civilians. Speaking to editors in New York, Iran’s President Hassan Rouhani stopped short of endorsing or condemning the airstrikes by the United States and Arab allies, though he raised questions about its legality.