Washington, United States: US President Donald Trump said Tuesday that a US military helicopter was shot down by Iran and that the United States "must" respond.

In a statement, Trump said he had been informed "that last night the Iranians shot down one of our highly sophisticated Apache Helicopters while patrolling over the Strait of Hormuz."

While the pilots were uninjured, "the United States must, of necessity, respond to this attack."

Meanwhile, a drone boat rescued the crew of a U.S. Army attack helicopter that crashed early Tuesday near the Strait of Hormuz, the strategic waterway that Iran has effectively closed during its war with the U.S. and Israel, a U.S. military official said.

It was the first known drone rescue at sea by the U.S. military, said Capt. Tim Hawkins, a spokesman for U.S. Central Command. He said the unmanned boat located the two aviators and brought them to shore after they spent about two hours in the water.

Military officials have not said what caused the Apache helicopter to go down. A military news release said the crash is under investigation.

The crash occurred with the Middle East still reeling after Iran and Israel exchanged fire the previous day in the biggest blow yet to the strained ceasefire in the Iran war. Iranian state television reported Tuesday that the Israeli attacks killed at least two members of the country’s air-defence units.

Since the U.S. and Israel began striking Iran on Feb. 28, the war has shaken the global economy, driven up energy prices around the world and made many basics, including food, more expensive.