US Secretary of State Mike Pompeo said the attack on the Taji base that killed three members of the US-led coalition in Iraq "will not be tolerated," in a tweet on Wednesday.
"Today's deadly attack on Iraq's Camp Taji military base will not be tolerated," Pompeo wrote.
He added that he and Britain's Foreign Secretary Dominic Raab agree that "those responsible must be held accountable."
The US Central Command said 18 Katyusha rockets struck the base outside of Baghdad, which houses US coalition troops.