U.S. officials said on Sunday 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 U.S. interests.

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

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

Iraqi security and militia sources said at least 18 militia fighters were killed and more than 50 wounded following three U.S. 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.

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

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

"We discussed with him other options that are available," Esper said. "I would note also that we will take additional actions as necessary to ensure that we act in our own self-defense and we deter further bad behavior 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 U.S. 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 Shiite 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 U.S. 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 U.S. civilian contractor and injured four U.S. 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, U.S. 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, Pompeo blamed Iranian-backed forces for a series of attacks on bases in Iraq and warned Iran that any attacks by Tehran or proxies that harmed Americans or allies would be "answered with a decisive U.S. response."

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

Iran says U.S. strikes on Iraq show its 'support for terrorism' 

On Monday, Iran said that the United States has shown its "support for terrorism" by carrying out airstrikes on forces in Iraq that have dealt blows against the ISIL.  

"These attacks have once again proved America's false claims in fighting Daesh (ISIL) ... as the United States has targeted the positions of forces that over the years have inflicted heavy blows to Daesh (ISIL) terrorists," Iran's government spokesman Abbas Mousavi said.  

"With these attacks, America has shown its firm support for terrorism and its neglect for the independence and sovereignty of countries, and it must accept consequences for its illegal act," he said in a statement.  

The spokesman said the presence of foreign forces in the region was the cause of insecurity and tensions. "America must put an end to its occupying presence," said Mousavi.  

(Input from Reuters, AFP)