At least seven mortar shells struck an Iraqi air base north of Baghdad, wounding four Iraqi soldiers.
Some of the shells hit the runway of the Balad air base, where some U.S. military personnel are based.
There was no immediate claim of responsibility for the attack, which comes amid heightened tensions between the United States and Iran.
Military bases hosting U.S. troops have been targeted by rocket and mortar attacks from Iran-backed militias in recent months. One U.S. contractor was killed in such an attack in December.
On January 8, Iran launched missile attacks against two such bases in Iraq in retaliation for an earlier U.S. drone strike that killed top Iranian military commander, Major General Qasem Soleimani.