The attack injured five US military personnel and two contractors.
Such attacks were common at the beginning of the war between Israel and Hamas.
The Ain al-Assad base houses US troops and other personnel.
The United States will not tolerate attacks on American troops in the Middle East, Defense Secretary Lloyd Austin said Tuesday, after rocket fire in Iraq wounded seven American personnel.
Monday's attack was the third in just over three weeks against the Ain al-Assad base in western Iraq, which houses US troops as well as other personnel from the US-led coalition against the Daesh group.
“Make no mistake, the United States will not tolerate attacks against our personnel in the region,” Austin said at a news conference in Annapolis.
A U.S. defense official said earlier in the day that the attack wounded five U.S. service members and two contractors, all of whom were in stable condition.
Such attacks were common at the beginning of the war between Israel and the Palestinian Hamas in Gaza, but had largely ceased until recently.
The latest rocket fire comes as fears grow of an attack by Iran and its allies against Israel in retaliation for the killing of senior Hamas and Hezbollah figures in attacks attributed to or claimed by Israel.
The killings are part of the most serious series of tit-for-tat attacks that have raised fears of a regional conflagration stemming from the Gaza war.
