At least three people were killed, including two US soldiers, and 10 wounded after rockets were fired at the Taji Camp in Iraq that houses international forces.
US officials quoted by media said on Wednesday that another of the dead was a British national.
The rocket attack was the 22nd against American military interests in the country since late October, an Iraqi military commander said.
US Army Colonel Myles Caggins, a US military spokesman in Iraq, said on Twitter more than 15 small rockets hit the base but provided no further details.
One American official, speaking to The Associated Press on condition of anonymity, said more information would be given later in an official announcement.
Camp Taji, located just north of Baghdad, has been used as a training base for a number of years. There are as many as 6,000 US troops in Iraq, training and advising Iraqi forces and conducting counter-terrorism missions.
Previous rocket attacks targeting US soldiers, diplomats and facilities in Iraq killed one US contractor and an Iraqi soldier. None of the attacks has been claimed, but Washington accuses pro-Iran factions of being responsible.
Two days after the death of an American in rockets fired on an Iraqi military base in Kirkuk at the end of last year, the US army hit five bases in Iraq and Syria used by the pro-Iran armed faction Kataeb Hezbollah.
Kataeb Hezbollah was designated a “foreign terrorist organization” by the State Department in 2009.
Tensions then rose further between foes Washington and Tehran after a US drone strike killed the powerful Iranian military commander Qasem Soleimani and an Iraqi paramilitary commander in Baghdad on January 3.
The assassination brought the two countries to the brink of war.
The US leads an international coalition – comprised of dozens of countries and thousands of soldiers – formed in Iraq in 2014 to confront the armed group ISIL (ISIS).
While ISIL has lost the vast territory it once held in Iraq and Syria, sleeper cells remain capable of carrying out attacks.
The Iraqi parliament voted to expel all foreign soldiers from the country in the wake of the killing of Soleimani, a decision that has still not been acted on by the government.
The outgoing government, which resigned in December in the face of mass protests, has yet to be replaced because of a lack of agreement in parliament – one of the most divided in Iraq’s recent history.