An Iranian-backed militia has vowed revenge for U.S. military strikes in Iraq and Syria that killed 25 militants.
The United States said on December 29 that it had conducted "precision defensive strikes" against five targets in Iraq and Syria controlled by the Kataib Hizbullah, also known as the Kataib Brigades.
The caretaker Iraqi government in power since Prime Minister Adil Abdul-Mahdi's resignation, meanwhile, warned on December 30 it might "review" its relations with the United States.
"American forces acted on their political priorities, not those of the Iraqis," AFP quoted the government as saying in a statement.
An unnamed senior official at the U.S. State Department said later of Iraqi authorities that it was "their responsibility and duty to protect us. And they have not taken the appropriate steps to do so," according to AFP.
"We have warned the Iraqi government many times, and we've shared information with them to try to work with them to carry out their responsibility to protect us as their invited guests," the agency said the official told reporters in Washington.
The Pentagon said three of the locations it struck were in Iraq and two in Syria, and that the strikes were in response to repeated Kataib Hizbullah attacks on Iraqi bases that house U.S. troops.
It singled out a December 27 rocket barrage in the northern city of Kirkuk that killed one defense contractor and injured four U.S. service members and two members of the Iraqi Security Forces.
Kataib Hizbullah, which said on December 30 that 25 of its fighters were killed by the U.S. strikes, has denied being behind the attacks.
"Our battle with America and its mercenaries is now open to all possibilities," the militia said in a statement released late on December 29. "We have no alternative today other than confrontation and there is nothing that will prevent us from responding to this crime."
Iraqi Shi'ite cleric Muqtada al-Sadr, who carries considerable influence over a leading bloc in the country's parliament, in a statement on December 30 urged Iranian-backed militias to avoid "irresponsible actions," according to Reuters.
He also said he was willing to cooperate with Iran-backed rivals to expel U.S. forces from Iraq, the agency said.
Sadr accused U.S. President Donald Trump of exploiting corruption in Iraq and the Persian Gulf region.
The Russian Foreign Ministry on December 30 called the "exchange of strikes" in Iraq "unacceptable," according to AFP.
"We consider such actions unacceptable and counterproductive," the Russian statement quoted by AFP said. "We call upon all parties to refrain from further actions that could sharply destabilize the military-political situation in Iraq, Syria, and the neighboring countries."
U.S. Defense Secretary Mark Esper said that F-15 Strike Eagle fighter jets were employed in the attacks that targeted "either command-and-control facilities or weapons caches for Kataib Hizbullah."
He said the "strikes were successful" and he didn't rule out additional action "as necessary," according to a press briefing Esper and U.S. Secretary of State Mike Pompeo gave in Florida, where Trump is on holiday.
The strikes "will degrade Kataib Hizbullah's ability to conduct future attacks against OIR [Operation Inherent Resolve] coalition forces," said a statement from Jonathan Hoffman, U.S. assistant to the secretary of defense, that was posted on the Pentagon's website.
Pompeo said that Trump had been briefed on the U.S. air strikes.
"We will not stand for the Islamic Republic of Iran to take actions that put American men and women in jeopardy," Pompeo told reporters after the briefing, which took place at Trump's Mar-a-Lago resort in Palm Beach.
Hoffman's statement cited what it said was "a strong linkage" to the Quds Force, an elite unit of Iran's Islamic Revolutionary Guards Corps (IRGC) responsible for foreign operations.
Later on December 29, AFP quoted "a security official" as saying that "four Katyusha rockets" had exploded in the evening near a base outside Baghdad that houses U.S. troops. There were no casualties, the agency quoted the unnamed source as saying.
Pompeo earlier this month accused Iranian-backed forces of a series of attacks on bases in Iraq and threatened "a decisive U.S. response" to any Iranian or Iranian-proxy attacks on U.S. nationals or those of its allies, according to Reuters.
Tensions have escalated between Tehran and Washington since the United States last year withdrew from an Iranian nuclear agreement with world powers and embarked on a "maximum-pressure" policy, including tough sanctions, to change Iranian behavior and force it back to the negotiating table.
Iranian officials have said they won't return to talks before sanctions are lifted.
In January, Pompeo said there were some 5,000 U.S. troops in Iraq.
There are around 1,000 U.S. troops in Syria.
U.S. and Saudi officials blamed Iran for aerial attacks on Saudi oil facilities at Abqaiq and Khurais in September that affected international oil supplies.
The United States has since sent hundreds more of its troops to Saudi Arabia to defend its close ally.
Iraq has been shaken by major anti-government protests for months that have been blamed largely on dissatisfaction with official corruption and incompetence but also have targeted the political system imposed after the U.S.-led invasion in 2003 as well as Iranian influence among Iraq's political elite.