U.S. forces struck Iranian rocket launchers on the Strait of Hormuz in the first military action in a month on Sunday, according to a U.S. official, breaking a lull in fighting during an intermittent war that has lasted more than six months.

Forces with the Revolutionary Guard Corps were observed preparing to launch rockets with sea mines into the strait, according to the official, who spoke on condition of anonymity to detail sensitive military movements.

The U.S. military last week completed clearing sea mines from the strait’s international shipping routes.