In a briefing with reporters, Pentagon press secretary John Kirby said the fighters dropped seven guided munitions at a facility he described as a "terrorist entry control point."
Kirby said the location was known to be used by Iranian-aligned militias, including Kaiti'b Hezbollah and Kaiti'b Sayyid al-Shuhada. The bombings completely destroyed nine facilities, he said, adding that two others were partially destroyed and left unusable.
Kirby said the airstrikes, which President Joe Biden authorized Thursday morning, were defensive in nature and in response to recent rocket attacks targeting American and coalition personnel in Iraq, as well as to "ongoing threats" against them.
The airstrikes represent the first major military operation under the new Biden administration, according to Kirby.
He said the military has early details about casualties at the site of the attack, but declined to provide further information, citing a battle damage assessment that is still under way.
The U.K.-based monitoring group Syrian Observatory on Human Rights said Friday that 22 militia fighters were killed in the attack. The group also said a weapons shipment was struck by U.S. fighters, and more militia members were wounded.
The targets were struck to damage the groups and their ability to conduct future attacks, Kirby said, though he would not discuss specific intelligence assessments. But the airstrikes were also intended to "send a very clear signal" that the U.S. will act to protect its people, he said.
"As we made clear last night -- and I think through President Biden's order, he made clear -- the United States will act to protect American and coalition personnel, and our security interests in the region," Kirby said. "These targets were chosen carefully, very deliberately, and struck in exactly the same manner."
Some Democratic and Democratic-leaning lawmakers questioned the legality of the strikes and the basis for their authorization. Sen. Tim Kaine, D-Va., who has pushed to repeal and narrow the Authorizations for the Use of Military Force passed after 9/11 and before the Iraq War, demanded that lawmakers be fully briefed on the strikes.
"The American people deserve to hear the administration's rationale for these strikes and its legal justification for acting before coming to Congress," Kaine said in a statement Friday.
And Sen. Bernie Sanders, I-Vermont., said he is concerned the attack "puts our country on the path of continuing the Forever War instead of ending it."
"This is the same path we've been on for two decades," Sanders said Friday. "For far too long, administrations of both parties have interpreted their authorities in an extremely expansive way to continue military interventions across the Middle East region and elsewhere. This must end."
Kirby said the Biden administration notified congressional leaders before the strikes and had been briefing lawmakers and their staffs Friday. The administration will hold a full classified briefing early next week, he said.
Kirby cited Article 2 of the Constitution, which spells out the president's powers as commander in chief of the armed forces, as giving Biden "not only the authority, but the obligation to protect American forces in combat theaters and in military operations."
He also pointed to Article 51 of United Nations International Law, which gives nations the right to self-defense.
"This really was a defensive strike, meant to help protect in the future American forces and coalition partners, given what we knew those structures were used for, right there on the other side of that border, to provide throughput for these groups and their activities right there inside Iraq," Kirby said.
The Pentagon is confident those facilities were used by the groups that attacked Americans and coalition forces in recent rocket attacks in Irbil in northern Iraq, and on the U.S. Embassy in Baghdad, making them legitimate targets, he said.
He referred to the structures as having "housing capabilities." When asked whether weapons were destroyed in the strikes, he declined to comment before the damage assessment was finished.
Kirby also said the U.S. alerted Russian forces in the area shortly before the airstrikes, using a deconfliction line set up to ensure the rival nations do not accidentally strike one another's forces.