The Iraqi military says three mortar shells have hit an air base just north of Baghdad where American trainers are present, causing a small fire but no casualties.

The military statement says the attack on Balad air base occurred early Saturday.

The attack comes amid rising tension in the Middle East between the United States and Iran, which ratcheted up on Thursday after suspected attacks on two oil tankers near the Strait of Hormuz. Iran has denied involvement.

Last month, a rocket exploded less than a mile away from the U.S. Embassy in Baghdad's heavily fortified Green Zone.

The current crisis is rooted in the U.S. withdrawal last year from the 2015 nuclear deal between Tehran and world powers. Washington subsequently re-imposed sanctions on Iran, sending its economy into freefall.