US officials say they believe Iran was behind the drone attack last week at the military outpost in southern Syria where American troops are based.

Officials said Monday the US believes that Iran resourced and encouraged the attack, but that the drones were not launched from Iran. They were Iranian drones, and Iran appears to have facilitated their use, officials said, speaking on condition of anonymity to discuss details that have not been made public.

Officials said they believe the attacks involved as many as five drones laden with explosive charges, and that they hit both the U.S. side of al-Tanf garrison and the side where Syrian opposition forces stay.

Pentagon spokesman John Kirby declined to provide details when asked about the report during a news conference Monday. He called it a “complex, coordinated and deliberate attack” and said the US has seen similar ones before from Shia militia groups that are backed by Iran. But he would not go into specifics and said he had no update on the munitions used in the attack.

Kirby also declined to say if troops were warned ahead of time or whether the US intends to make a military response.

“The protection and security of our troops overseas remains a paramount concern for the secretary,” Kirby said, referring to Defense Secretary Lloyd Austin, “and that if there is to be a response, it will be at a time and a place and a manner of our choosing, and we certainly won't get ahead of those kinds of decisions.