The Iraqi government has strongly condemned US strikes which targeted sites used by Iranian-backed groups in Iraq on Wednesday.
A spokesperson for Iraqi Prime Minister Mohammed Shia al-Sudani said they "blatantly" violated his country's sovereignty.
The US said its "proportionate" attacks had targeted "Iran-affiliated groups".
The paramilitary Popular Mobilisation Forces (PMF) said the "treacherous" US action killed one of their fighters.
The PMF, which is dominated by Iran-backed Shia Muslim militias, said a number of other fighters were injured in strikes on their bases in al-Qaim, a town on the Syrian border in western Anbar province, and in Jurf al-Nasr, in the central province of Babil.
US Defence Secretary Lloyd Austin said the strikes on three facilities belonging to the Kataib Hezbollah militia and other groups were "in direct response to a series of escalatory attacks" against US and other international forces in Iraq and Syria.
Major General Yehia Rasool, a spokesman for Prime Minister Sudani, said in a statement that the US action was "contributing to a reckless escalation".
"This unacceptable act undermines years of co-operation... at a time when the region is already grappling with the danger of expanding conflict, the repercussions of the aggression on Gaza," he added, referring to the war between Israel and the Palestinian group Hamas.
He added that Iraq would treat the US operations "as acts of aggression" against its people on their land, and urged the international community to help restore peace.
Writing on X, formerly Twitter, Iraq national security adviser Qassem al-Aaraji said the US action would "not help bring calm".
He added that "the US should pile on pressure for a halt to the Israeli offensive in Gaza rather than targeting and bombing the bases of an Iraqi national body".
Last week, four US military personnel were injured in a ballistic missile and rocket attack on Iraq's Al Asad air base, in Anbar province.
The Pentagon said on Tuesday that they had returned to duty and that there was no significant damage to facilities.
The US military's Central Command (Centcom) said an Iran-backed militia targeted the air base, which hosts American troops.
The Islamic Resistance in Iraq claimed it was behind that attack.
The umbrella group emerged in late 2023 and is comprised of several Iran-affiliated militias operating in Iraq. It has claimed other attacks against US forces in recent weeks.
Mr Austin said: "We do not seek to escalate conflict in the region. We are fully prepared to take further measures to protect our people and our facilities."
"We call on these groups and their Iranian sponsors to immediately cease these attacks."
In a separate statement, Centcom said the strikes were carried out at 00:15 local time on Wednesday (21:15 GMT Tuesday).
"These strikes targeted [Kataib Hezbollah] headquarters, storage, and training locations for rocket, missile, and one-way attack UAV capabilities," it added, without mentioning their locations.
Kataib Hezbollah, or Brigades of the Party of God, is a powerful Iraqi Shia militia that receives financial and military support from Iran.
It is believed to have strong links with Iran's Quds Force, the overseas operations arm of the Islamic Revolution Guard Corps.
Since 2009, the US has designated the group as a terrorist organisation, accusing it of attacking US and Iraqi forces in Iraq on behalf of Iran, and of threatening Iraq's stability.
Saturday's attack on Al Asad air base followed a US drone strike in Baghdad earlier this month in which a high-ranking PMF commander was killed.
US troops in Iraq and Syria have been attacked dozens of times by Iran-aligned fighters since the war in Gaza started in October.
The US and UK have also carried out strikes on Yemen's Iran-backed Houthi movement in response to its missile and drone attacks on international shipping in the Red Sea and Gulf of Aden.
Meanwhile, Iran has carried out a number of missile strikes over the last week against targets in Syria, Iraq and Pakistan, which it claimed were linked to the Islamic State group, Israel and a Baloch separatist group respectively.
Last Saturday, Iran accused Israel of carrying out an air strike in the Syrian capital, Damascus, which killed five senior members of Iran's security forces.