The United States launched airstrikes in Iraq early Friday, responding to a rocket attack on a military base that killed one British and two U.S. service members.

The Pentagon said in a statement that the strikes targeted facilities "across Iraq" controlled by Kitaeb Hezbollah, a militia that U.S. officials say has attacked American troops before. The targets included storage facilities that housed weapons used to target U.S. and coalition troops, the statement said.

The Pentagon described the strikes as "defensive, proportional, and in direct response to the threat" posed by Iranian-backed militias.

"The United States will not tolerate attacks against our people, our interests, or our allies," Defense Secretary Mark Esper said in the statement. "As we have demonstrated in recent months, we will take any action necessary to protect our forces in Iraq and the region."

The aerial bombardment took place around 1:30 a.m., according to the Iraqi military. It was unclear whether any militia members were killed. In a statement early Friday, another Iran-backed militia, Harakat al-Nujaba, accused the "reckless and arrogant" United States of hitting militia and Iraqi army headquarters, as well as a civilian airport. The militia said that further strikes could prompt retaliation involving an "eye for an eye." The group did not elaborate on what that might be, and it was not immediately possible to confirm the U.S. statements that the strikes had only targeted militia facilities, or the militia claim that a civilian-related location had been struck.

U.S. defense officials had said Iranian-backed militia groups had carried out the attack Wednesday on Camp Taji, an Iraqi base north of Baghdad occupied by coalition forces.

Army Gen. Mark Milley, the chairman of the Joint Chiefs of Staff, told reporters at the Pentagon on Thursday morning that defense officials had "pretty good confidence" who launched the rockets, citing the capture of a truck by Iraqi security forces on the northern outskirts of Baghdad they said was used in the attack. Iraqi officials released photos of the truck, a flatbed with launchers rigged to it in makeshift fashion.

"You don't get to shoot at our bases and kill and wound Americans and get away with it," Esper told reporters.

Milley said that about 30 rockets were fired at Taji, a base that is shared by Iraqi and coalition forces. Twelve to 18 rockets landed on the base, wounding 14 people, including five seriously, and causing some structural damage, the general said. The wounded included U.S., British and Polish service members, as well as contractors, Milley said.

The Pentagon was withholding the names of the Americans killed until families were notified.

The British Ministry of Defense identified its service member killed as Lance Cpl. Brodie Gillon, 26. She was a reservist and combat medical technician with the Scottish and North Irish Yeomanry and deployed with the Irish Guards Battle Group, the ministry said.

The rocket attack and U.S. retaliation mark the latest shots fired in a proxy war with Iran that stretches back years and escalated amid the Trump administration's "maximum pressure" campaign, in which crippling sanctions have been placed on Iran in an attempt to get it to negotiate a new deal banning it from making nuclear weapons.

On Dec. 27, a similar rocket attack on a base outside the city of Kirkuk, killing a U.S. interpreter, Nawres Hamid. Two days later, U.S. forces launched airstrikes on five sites in Iraq and Syria affiliated with the militia Kitaeb Hezbollah. U.S. officials have long said that they are supplied by Iran's Quds Force.

Militia members and supporters responded by storming entrances to the U.S. Embassy in Baghdad on Dec. 31, chanting "Death to America" and setting fires. They eventually withdrew.

On Jan. 3, the United States launched a drone strike near the Baghdad International Airport that killed Qasem Soleimani, the Iranian general whose Quds Force supplies arms to Kitaeb Hezbollah and other militias, according to U.S. officials.

That spawned concerns that Iran might retaliate with a larger attack in the region, including potentially at sea. On Jan. 8, Iran launched ballistic missiles at two military bases in Iraq. No U.S. or Iraqi troops were killed, but the Pentagon reported more than 100 service members suffered concussive injuries.

Rockets have been fired at bases in Iraq since then, but no Americans had been killed.

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The Washington Post's Mustafa Salim in Baghdad contributed to this report.

This article was written by Dan Lamothe and Louisa Loveluck, reporters for The Washington Post.