President Donald Trump blamed Iran for shooting down a U.S. Army helicopter near the Strait of Hormuz on Tuesday and said the United States must respond to the attack.

In the first known operation of its kind by the American military, a drone boat rescued the two aviators who were aboard the Apache attack helicopter when it went down near the critical shipping lane that Iran has effectively closed during its war with the U.S. and Israel.

Trump said in a social media post that military officials told him “the Iranians shot down one of our highly sophisticated Apache Helicopters.” He added that both service members “are safe and uninjured.”

“Nevertheless, the United States must, of necessity, respond to this attack,” Trump wrote.

The downing of the helicopter further strained a two-month ceasefire a day after Iran and Israel exchanged fire for the first time since the fragile truce took effect. Iranian state television said Tuesday that the Israeli attacks killed at least two members of the country’s air-defense units.

Read the full report from The Associated Press.