The Apache helicopter is the second crewed aircraft that Washington has confirmed was shot down by Iran during the Middle East war, following the loss of an F-15 fighter plane in April.

The downing and the prospect of a US response pose the latest of a series of threats to a shaky ceasefire that has been in place since April 8, as the United States and Iran struggle to negotiate an end to the war.

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 crew members were uninjured, "the United States must, of necessity, respond to this attack."

The Apache is an attack helicopter with a crew of two that is armed with a 30mm chain gun and can carry various other weapons including Hellfire missiles.

US Central Command (CENTCOM), which is responsible for US forces in the Middle East, said earlier that two Apache crew members "were rescued by American forces after their helicopter went down near the coast of Oman."

"The Soldiers were safely rescued within approximately two hours and are in stable condition," CENTCOM said in a post on X.

The command also said a naval surface drone helped rescue the downed helicopter's crew.