The United States said on December 29 that it had bombed five targets in Iraq and Syria controlled by an Iraqi Shi'a paramilitary group with suspected ties to Iran's elite Quds Force.
The Pentagon said three of the Kata'ib Hizbollah locations it struck were in Iraq and two in Syria.
Iraqi military and security sources said three U.S. air strikes on December 29 had killed at least 18 militia fighters and wounded more than 50, Reuters said. They cited one attack, likely carried out by drones, that appeared to target a militia's headquarters near the western Qaim district of Iraq that borders Syria.
"In response to repeated Kata'ib Hizbollah (KH) attacks on Iraqi bases that host Operation Inherent Resolve (OIR) coalition forces, U.S. forces have conducted precision defensive strikes against five KH facilities in Iraq and Syria that will degrade KH's ability to conduct future attacks against OIR coalition forces," a statement From U.S. Assistant to the Secretary of Defense Jonathan Hoffman posted on the Pentagon's website said.
The U.S. statement blamed the Kata'ib Hizbollah, also known as the Kata'ib Brigades, for recent attacks including a "30-plus rocket attack on an Iraqi base near Kirkuk that resulted in the death of a U.S. citizen and injured four U.S. service members and two members of the Iraqi Security Forces (ISF)."
"These locations included weapon storage facilities and command-and-control locations that KH uses to plan and execute attacks on OIR coalition forces," Hoffman's statement said.
It cited what it said was "a strong linkage" to the Quds Force, a unit of Iran's Islamic Revolutionary Guards Corps (IRGC) responsible for foreign operations.
Later on December 29, AFP quoted "a security official" as saying that "four Katyusha rockets" had exploded in the evening near a base outside Baghdad that houses U.S. troops. There were no casualties, the agency quoted the unnamed source as saying.
U.S. Secretary of State Mike Pompeo earlier this month accused Iranian-backed forces of a series of attacks on bases in Iraq and threatened "a decisive U.S. response" to any Iranian or Iranian-proxy attacks on U.S. nationals or those of its allies, according to Reuters.
Tensions have escalated between Tehran and Washington since the United States last year withdrew from an Iranian nuclear agreement with world powers and embarked on a "maximum-pressure" policy, including tough sanctions, to change Iranian behavior and force it back to the negotiating table.
Iranian officials have said they won't return to talks before sanctions are lifted.
In January, Pompeo said there were some 5,000 U.S. troops in Iraq.
There are around 1,000 U.S. troops in Syria.
U.S. and Saudi officials blamed Iran for aerial attacks on Saudi oil facilities at Abqaiq and Khurais in September that affected international oil supplies.
The United States has since sent hundreds more of its troops to Saudi Arabia to defend its close ally.
Iraq has been shaken by major anti-government protests for months that have been blamed largely on dissatisfaction with official corruption and incompetence but also have targeted the political system imposed after the U.S.-led invasion in 2003 as well as Iranian influence among Iraq's political elite.