The head of U.S. forces in the Middle East says an Iranian-backed militia likely launched the attack killing the two Americans and a British soldier in Iraq Wednesday night.

"The Iranian proxy group Kata'eb Hezbollah is the only group known to have previously conducted an indirect fire attack of this scale against U.S. and coalition forces in Iraq," Gen. Kenneth McKenzie told Senate lawmakers Thursday morning.

Despite adding over 10,000 U.S. troops to the region since May, McKenzie says Iran continues its attacks against U.S. forces.

“What has not changed is their continuing desire to operate through their proxies indirectly against us,” McKenzie added.

ROCKET FIRE HITS BASE IN IRAQ HOUSING US TROOPS, KILLING 2 AMERICANS, 1 BRITON, MILITARY SAYS

Iranian-backed Shia militia launched 18 Katyusha rockets at Camp Taji, located 17 miles north of Baghdad -- the largest attack on U.S. troops since Iran fired ballistic missiles in early January, days after a U.S. drone strike that killed Iran's most powerful general Qassem Soleimani at Baghdad's airport. Wednesday would have reportedly been Soleimani's 63rd birthday, the same day as the attack.

U.S. Central Command says 12 additional coalition troops were wounded in the rocket attack. Iraqi forces found a truck rigged with rocket-launching tubes a few miles from the base.

Secretary of State Mike Pompeo spoke to his British counterpart after the attack.

2 US SERVICE MEMBERS KILLED IN IRAQ DURING MISSION AGAINST ISIS

"Today’s deadly attack on Iraq’s Camp Taji military base will not be tolerated. @DominicRaab and I agree – those responsible must be held accountable," Pompeo said in a tweet.

Ben Wallace, Britain's defense secretary, said Thursday: “Last night’s attack on UK and coalition personnel was a cowardly and retrograde act. The men and women of the UK armed forces are in Iraq to help that country establish stability and prosperity. The people that did this are not friends of Iraq."

Wednesday night, the remains of two U.S. Marine special operators who were killed Sunday in Iraq battling ISIS, returned to Dover Air Force base. Capt. Moises A. Navas, of Germantown, Md., and Gunnery Sgt. Diego D. Pongo, of Simi Valley, Calif. were both 34 years old.

McKenzie did not drop any hints about a potential military response to the latest attack in Iraq , but told lawmakers he now has two aircraft carrier strike groups in the Middle East, the USS Harry Truman and USS Dwight D. Eisenhower.

In late December, the U.S. military blamed Kata'eb Hezbollah--whom the State Department designates a terrorist group--for launching a rocket attack that killed an American contractor.

Days later the U.S. military launched an airstrike targeting Kata’eb Hezbollah’s camps in western Iraq and eastern Syria killing dozens of militants.

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The terrorist group responded by sending an angry mob to the U.S. embassy in Baghdad on New Year’s Eve.