Iraq condemned the United States on Monday for launching airstrikes against an Iranian-backed militia on its soil and warned the action could damage relations.
The country’s National Security Council said it would have to reconsider working with the US-led coalition against Islamic State.
US, British and other foreign troops are only able to operate in Iraq with the permission of the government. Any review of the relationship could lead to permission being withdrawn.
The warning came as US forces in Iraq were on alert for possible reprisal attacks war planes targeted the Kataib Hezbollah militia group on Sunday night in an escalation of tensions.
A military officer told the Reuters news agency: “I think they will retaliate.”
The founder of the militia – who is closely aligned with Iran – vowed revenge.
“The blood of the martyrs will not be in vain and our response will be very tough on the American forces in Iraq,” Jamal Jaafar Ibrahimi, who is known by his nom de guerre Abu Mahdi al Mohandes, said in a statement.
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Iraq’s embattled government condemned the US action against the state-approved militia group in western Iraq on Sunday as a “flagrant violation” of its sovereignty.
Iran, which is locked in an increasingly bitter standoff with the United States, accused Washington of “an act of terrorism” over the attacks, even as US officials warned they were ready to launch more offensive action to defend US personnel and interests if required.
Concerns are growing that Iraq will become a new battleground for Tehran and Washington to wage a proxy war
In a statement from his office, Prime Minister Adel Abdul Mehdi “described the American attack on the Iraqi armed forces as an unacceptable vicious assault that will have dangerous consequences”.
But US officials said any violation of sovereignty had been carried out by the Iranian-backed militia when it allegedly targeted a base in the northern city of Kirkuk in a rocket attack that left a US contractor dead.
US Assistant Secretary of State David Schenker said the Sunday strikes on were a message to Iran after months of “restraint” by the US administration.
“We thought it important to hit a significant target set to send a very clear message to them about how serious we take American lives,” Mr Schenker told reporters in a telephone conference.
“This was a response that was serious, but was, I think in many ways, proportionate,” he said. “We don’t want an escalation here, we want a de-escalation.”
Brian Hook, the US envoy on Iran, said Washington had held back from responding despite a series of attacks tied to Tehran-linked militias.
President Donald Trump “has been very patient. He has shown a great deal of restraint,” Mr Hook said in the same briefing.
“We very much hoped that Iran would not miscalculate and confuse our restraint for weakness. But after so many attacks, it was important for the president to direct our armed forces to respond in a way that the Iranian regime will understand.”
“We will not tolerate attacks against US citizens, our military or our partners and allies in the region,” Mr Hook added.
The Sunday night airstrikes are said to have killed at least 25 militia fighters, according to Iraqi sources.
The F-15 war planes struck three targets in Iraq and two across the border in Syria.
Kataib Hezbollah’s headquarters in the Iraqi border district of al Qaim was hit, along with other bases and weapons storage facilities.
Drone footage released by the Pentagon showed the impact of the strikes in western Iraq.
US officials said the move was in response to rocket attacks on a military base in the northern Iraqi city of Kirkuk on Friday, which left a US contractor dead.
US troops and Iranian-backed militias have largely avoided direct confrontation in Iraq over the past five years as they both fought a common enemy in Islamic State.
But a growing, global standoff between Washington and Tehran over Iran’s nuclear ambitions could see a flare up in hostilities on the ground in Iraq where both countries have a lot of interests, personnel and influence.
With Iraq’s government also in crisis after months of public protests, it is a particularly vulnerable time in an already unstable country.
More than 5,000 US military personnel are based in Iraq as well as some 400 British forces.
President Trump discussed the Iranian threat to forces in Iraq during a meeting with Mr Esper, US Secretary of State Mike Pompeo and General Mark Milley, the top US military officer, on Sunday at his Mar-a-Lago resort in Florida.
Speaking afterwards, Mr Pompeo accused Iranian-backed forces of having targeted facilities in Iraq housing US personnel for weeks.
“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,” he told reporters.
Iran has denied any involvement in such attacks.
“We strongly deny any role in the attack on American forces. This claim without any evidence cannot justify bombing and killing people in violation of international law,” said Iranian government spokesman Ali Rabiei, quoted by the semi-official news agency Fars.
A spokesman for the Iranian foreign ministry condemned the US airstrikes as an “obvious case of terrorism” and called on Washington to respect Iraq’s territorial integrity.
Speaking on a trip to Moscow, Iranian foreign minister Javad Zarif said: “The recent actions by Americans in Iraq are unacceptable.
“Americans do it, but Iran and Russia put efforts to establish peace both in Syria and in Iraq.”
Since October 28, at least 11 attacks have targeted Iraqi military bases where US military personnel or diplomats are deployed.
While earlier attacks killed an Iraqi soldier and wounded others, the one on Friday was the first to kill an American, targeting a meeting between Iraqi police commanders and the international coalition that fought IS.