The US carried out airstrikes against a pro-Iran militant group in Iraq and Syria, two days after a rocket attack on an Iraqi base killed a US civilian contractor.

The Pentagon said Sunday (December 29th) it targeted weapons caches or command and control facilities linked to Iran-backed Iraqi paramilitary group Kataib Hizbullah in western Iraq and eastern Syria.

The Sunday night attacks saw US planes hit three sites in Iraq and two in Syria belonging to Kataib Hizbullah.

The targeted sites included weapons storage facilities and command locations used to plan and execute attacks, the Pentagon said in a statement.

The strikes were in retaliation for a series of rocket attacks since late October against US interests in Iraq, including a barrage of more than 30 fired on Friday against K1 Iraqi military base in Kirkuk, which killed a US civilian contractor.

Four US service members and members of the Iraqi forces also were wounded.

The K1 base attack involved a direct hit on an ammunition depot that caused secondary explosions, and four more rockets were found in their tubes in a truck at the launch point, said a US official who spoke on condition of anonymity.

"In response to repeated Kataib Hizbullah attacks on Iraqi bases that host Operation Inherent Resolve (OIR) coalition forces, US forces have conducted precision defensive strikes against five Kataib Hizbullah facilities in Iraq and Syria that will degrade Kataib Hizbullah's ability to conduct future attacks against OIR coalition forces," a Pentagon spokesman said.

Secretary of State Mike Pompeo said the US would not stand for Iran "to take actions that put American men and women in jeopardy".

Defence Secretary Mark Esper said Sunday the airstrikes were successful, and did not rule out further action to "deter further bad behaviour from militia groups or from Iran".

Bahrain, a key US ally in the region, on Monday said it welcomed the strikes.

Strong links with IRGC-QF

Kataib Hizbullah "has a strong linkage" with the Quds Force of Iran's Islamic Revolutionary Guard Corps (IRGC-QF), the Pentagon said earlier.

The militia "has repeatedly received lethal aid and other support from Iran that it has used to attack" coalition forces, it said.

The Popular Mobilisation Forces, a paramilitary umbrella group in Iraq, on Monday said the Sunday US airstrikes in Iraq had killed at least 25 fighters.

The strikes "killed 25 and wounded 51, including commanders and fighters, and the toll could yet rise", the PMF said.

Shortly after the US airstrikes, four Katyusha rockets exploded Sunday night near Taji Iraqi military base outside Baghdad, which houses US troops, without causing casualties, said an Iraqi security official who did not want to be named.