WASHINGTON (AP) — President Donald Trump declared Thursday that "Iran made a very big mistake" by shooting down a U.S. surveillance drone over the Strait of Hormuz but suggested it was a foolish error rather than an intentional escalation of the tensions that have led to rising fears of open military conflict.

Asked about a U.S. response, the president said pointedly, "You'll soon find out."

The downing of the huge, unmanned aircraft , which Iran portrayed as a deliberate defense of its territory rather than a mistake, was a stark reminder of the risk of military conflict between U.S. and Iranian forces as the Trump administration combines a "maximum pressure" campaign of economic sanctions against Iran with a buildup of American forces in the region.

On Thursday, Iran called the sanctions "economic terrorism," insisted the drone had invaded its airspace and said it was taking its case to the United Nations in an effort to prove the U.S. was lying about the aircraft being over international waters. It accused the U.S. of "a very dangerous and provocative act."

The drone — which has a wingspan wider than a Boeing 737 — entered Iranian airspace "despite repeated radio warnings" and was shot down by Iran, acting under the U.N. Charter which allows self-defense action "if an armed attack occurs," Iran's U.N. Ambassador Majid Takht Ravanchi said in a letter to the U.N. secretary-general.