WASHINGTON/BAGHDAD (REUTERS, AP, NYTIMES) - United States officials said on Sunday (Dec 29) that air strikes in Iraq and Syria against an Iran-backed militia group were successful, but warned that "additional actions" may still be taken in the region to defend US interests.

The US military carried out the strikes on Sunday against the Kataib Hezbollah militia group in response to the killing of a US civilian contractor in a rocket attack on an Iraqi military base, officials said.

US President Donald Trump was briefed by his top national security advisers following the strikes at his Mar-a-Lago club in Palm Beach, Florida.

A US response to an attack that kills or wounds Americans is not unusual. But Sunday's retaliation involved direct strikes on Iranian proxies, making it particularly dangerous ground.

Since the US military returned to Iraq in 2014, Iranian-backed forces and US forces have refrained from attacking each other because of a common enemy: the Islamic State in Iraq and Syria (ISIS) group. But the ISIS has lost its territory, and tensions have risen between Teheran and Washington over the Trump administration's "maximum pressure" campaign.

"We will not stand for the Islamic Republic of Iran to take actions that put American men and women in jeopardy," US Secretary of State Mike Pompeo told reporters after the briefing with Mr Trump.

Mr Pompeo, Defence Secretary Mark Esper and General Mark Milley, chairman of the US Joint Chiefs of Staff, appeared briefly in a club ballroom to comment on the airstrikes.

Mr Esper termed the offensive "successful", but said that Mr Trump was informed that a further military response could be warranted.

"We discussed with him other options that are available," Mr Esper said. "I would note also that we will take additional actions as necessary to ensure that we act in our own self-defence and we deter further bad behaviour from militia groups or from Iran."

Iraqi security and militia sources said at least 25 militia fighters were killed and at least 55 wounded following three US air strikes in Iraq on Sunday.

At least four local Kataib Hezbollah commanders were among the dead, the sources said, adding that one of the strikes had targeted the militia group's headquarters near the western Qaim district on the border with Syria.

The Pentagon said it had targeted three locations of the Iranian-backed Shi'ite Muslim militia group in Iraq and two in Syria. The locations included weapons storage facilities and command and control locations the group had used to plan and execute attacks on coalition forces, it said.

A US official, speaking on the condition of anonymity, said the strikes were carried out by F-15 fighter jets.

The US had accused Kataib Hezbollah of carrying out a strike involving more than 30 rockets last Friday which killed the US civilian contractor and injured four US service members and two members of the Iraqi Security Forces near the oil-rich city of Kirkuk.

"In response to repeated Kataib Hizbollah attacks on Iraqi bases that host Operation Inherent Resolve (OIR) coalition forces, US forces have conducted precision defensive strikes... that will degrade KH's ability to conduct future attacks against OIR coalition forces," chief Pentagon spokesman Jonathan Hoffman said in a statement.

Earlier this month, Mr Pompeo blamed Iranian-backed forces for a series of attacks on bases in Iraq and warned Iran that any attacks by Teheran or proxies that harmed Americans or allies would be "answered with a decisive US response".

Tensions have heightened between Teheran and Washington since last year when Mr Trump pulled the US out of Teheran's 2015 nuclear deal with six powers and reimposed sanctions that have crippled Iran's economy.

Last Friday's militia strike and US counter-strike come as months of political turmoil roil Iraq. About 500 people have died in anti-government protests in recent months, most of them demonstrators killed by Iraqi security forces.

The mass uprisings prompted the resignation of Prime Minister Adel Abdul-Mahdi late last month. Mr Abdul-Mahdi remains for now in a caretaker capacity.

Mr Abdul-Mahdi had made no public comment on last Friday's militia attack but condemned the US retaliatory strike on Sunday. He called it a violation of Iraqi sovereignty and a "dangerous escalation that threatens the security of Iraq and the region".

In a statement, Mr Abdul-Mahdi said Mr Esper had called him about a half-hour before the US strikes to tell him of US intentions to hit bases of the militia suspected of being behind Friday's rocket attack.

Mr Abdul-Mahdi said in the statement that he asked Mr Esper to call off US retaliation plans. The statement said Iraqi President Barham Salih also received advance notice from a US diplomat, and also asked unsuccessfully for Americans to call it off.