WASHINGTON/BAGHDAD (REUTERS, AP, AFP, BLOOMBERG) - The US military carried out air strikes in Iraq and Syria 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, US officials said on Sunday (Dec 29).

Iraqi security and militia sources said at least 25 militia fighters were killed and some 55 were 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 United States had accused Kataib Hezbollah of carrying out a strike involving more than 30 rockets on 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.

US President Donald Trump was briefed on the strikes by his top national security advisers at the president's Mar-a-Lago club in Palm Beach, Florida, US Secretary of State Mike Pompeo said.

"What we did was take a decisive response that makes clear what President Trump has said for months and months and months, which is that we will not stand for the Islamic Republic of Iran to take actions that put American men and women in jeopardy," Mr Pompeo said.

Mr Pompeo and Defence Secretary Mark Esper flew to Florida to brief Mr Trump on activities in the past three days. .

"This wasn't the first attack against this particular Iraqi facility or others where there were American lives at risk," Mr Pompeo said.

Mr Esper said the strikes were successful and did not rule out addition actions in the region.

"We will take additional actions as necessary to ensure that we act in our own self defence and we deter other bad behaviour from militia groups," he said. Neither official took questions.

Iraq's Joint Operations Command said in a statement that three US airstrikes on Sunday evening Iraq time hit the headquarters of the Hezbollah Brigades at the Iraq-Syria border, killing four fighters.

Iraq's Hezbollah Brigades, a separate force from the Lebanese group Hezbollah, operate under the umbrella of the state-sanctioned militias known collectively as the Popular Mobilisation Forces. Many of them are supported by Iran.

The Popular Mobilisation Forces said Sunday that the US strikes killed at least 19 of Kataib Hezbollah's members.

Kataib Hezbollah is led by Abu Mahdi al-Muhandis, one of Iraq's most powerful men. He once battled US troops and is now the deputy head of the Popular Mobilisation Forces.

In 2009, the State Department linked him to the elite Quds Force of Iran's Revolutionary Guard, designated a foreign terrorist organisation by Mr Trump earlier this year.

The US maintains some 5,000 troops in Iraq. They are there based on an invitation by the Iraqi government to assist and train in the fight against the Islamic State of Iraq and Syria (ISIS) group.

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."

US-Iran tensions have soared since Washington pulled out of a landmark nuclear agreement with Teheran last year and imposed crippling sanctions.

Baghdad - which is close to both countries - risks being caught in the middle.

In Iraq's neighbour Syria, Shiite powerhouse Iran backs the government of President Bashar al-Assad in an eight-year civil war.

Friday's attack and the US retaliation come as Iraq is gripped by its biggest anti-government street protests since the US-led invasion of 2003 that toppled dictator Saddam Hussein.

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 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 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 off it off.

Another powerful pro-Iran faction, Assaib Ahl al-Haq - whose leaders were recently hit with US sanctions - called for Americans to withdraw from Iraq.

"The American military presence has become a burden for the Iraqi state and a source of threat against our forces," it said in a statement. "It is therefore imperative for all of us to do everything to expel them by all legitimate means."

Since Oct 28, at least 11 attacks have targeted Iraqi military bases where US soldiers or diplomats are deployed, including five rockets that hit Al-Asad air base on Dec 3, just four days after US vice-president Mike Pence visited troops there.

A US source has said pro-Iran factions in Iraq are now considered a more significant threat to American soldiers than ISIS, whose sweeping offensive in 2014 saw Washington deploy thousands of troops to the country.