Two Americans and one Briton were killed as approximately 18 Katyusha rockets hit an Iraqi base housing U.S. troops Wednesday, the U.S. military said in a statement.

The attack came just days after two U.S. service members were killed by enemy forces while advising and accompanying Iraqi Security Forces during a mission to eliminate an ISIS terrorist stronghold in a mountainous area of north-central Iraq on Sunday.

The U.S. military could not confirm who was responsible for Wednesday's attack but one source told Fox News that "ISIS doesn't have this capability."

2 US SERVICE MEMBERS KILLED IN IRAQ DURING MISSION AGAINST ISIS

"ISIS doesn't have this capability." — Source reacting to attack against U.S. troops in Iraq

"The Coalition @CJTFOIR confirms more than 15 small rockets impacted Iraq’s Camp Taji base hosting Coalition troops, March 11 at 7:35 p.m. (Iraq Time)," Operation Inherent Resolve (OIR) spokesperson Col. Myles Caggins wrote on Twitter.

Iraqi Security Forces found a rocket-rigged truck a few miles from Camp Taji, which hosts Coalition personnel for training and advising missions, according to a statement from U.S. Central Command.

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The Camp Taji attack was under investigation by the Coalition and Iraqi Security Forces, U.S. Central Command said.

At least 12 personnel were wounded during the attack, the statement added.

The attack was the largest on a U.S. base in Iraq since Iran fired ballistic missiles at bases housing U.S. troops in early January.

Fox News' Lucas Tomlinson contributed to this report.