US President Donald Trump has confirmed that Iran shot down a US military helicopter on Monday.

He said: "I have just been informed by our Great Military that last night the Iranians shot down one of our highly sophisticated Apache Helicopters while patrolling over the Strait of Hormuz. There were two pilots involved, both are safe and uninjured. Nevertheless, the United States must, of necessity, respond to this attack. Thank you for your attention to this matter! President DONALD J. TRUMP."

A drone boat rescued the crew of a US 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 had not confirmed what caused the Apache helicopter to go down.

AH-64 Apache helicopters have been a key asset for the American military as it enforces a blockade on Iranian crude oil shipments and tankers, seeking to pressure Tehran into a deal. The helicopters have also been used by the United Arab Emirates to shoot down Iranian drones.

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.

Officials have been unable to turn the April ceasefire into a deal to permanently end the conflict, particularly as Israel intensifies and expands its military campaign in Lebanon against the Iranian-backed militia Hezbollah.