The US says it has carried out air strikes against Iran-backed militias near the Iraq-Syria border in response to drone attacks on its forces in Iraq.
A Pentagon spokesman said the strikes targeted “operational and weapons storage facilities” at three locations.
President Biden had been clear he would act to protect US personnel, he added.
A powerful Iraqi militia alliance, the Popular Mobilisation Forces (PMF), said four members of one faction were killed and threatened to retaliate.
About 2,500 US troops are in Iraq as part of a global coalition supporting local security forces in their fight against the jihadist group, Islamic State (IS).
There have been at least five drone attacks against facilities used by US and coalition personnel since April, US officials say. Rockets have also been fired at them and supply convoys have been targeted by improvised explosive devices.
How have the militias responded?
The PMF – a coalition of mostly Iranian-backed Shia militias that was formed in 2014 to fight IS, and was incorporated into the Iraqi Security Forces three years ago – said the strikes killed four fighters carrying out an officially-sanctioned mission to prevent IS militants infiltrating Iraq.
It added that the fighters had “not [been] involved in any activity against the foreign presence in Iraq”, and that the targeted locations did “not include any [weapon] stores or similar, in contrast to the claims made by the US”.
“As such, we denounce and condemn in the strongest terms this sinful attack on our forces… and we affirm that we maintain the legal right to respond to these attacks and hold accountable their perpetrators on Iraqi territory.”
Iraqi military spokesman Maj-Gen Yehia Rasool also condemned the strikes, writing on Twitter that they represented “a blatant and unacceptable violation of Iraqi sovereignty and Iraqi national security”.
He also reiterated Iraq’s “rejection of becoming a field for settling scores” and called for “calm and the avoidance of escalation in all forms”.
Iranian foreign ministry spokesman Saeed Khatibzadeh said the US was “destroying security” in the Middle East and should “refrain from taking emotional steps, creating crises and tensions, and multiplying problems for people in the region”.
Syrian state media reported that a child was killed in a strike near the Syrian border town of Albu Kamal early on Monday, and accused the US of seeking to undermine efforts to improve stability in the area.
The Syrian government has relied heavily on support from Iran’s Revolutionary Guards and Shia militias during the country’s civil war, and they have a significant presence in the border region.
The Syrian Observatory for Human Rights, a UK-based monitoring group, said seven militia fighters were killed in strikes near Albu Kamal.