The Pentagon has said the US has carried out retaliatory targeted air strikes against "facilities used by Iran-backed militia groups" on the Iraq-Syria border, in which at least five militia fighters are reported to have been killed.
The US has blamed Iran-linked Iraqi factions for recent attacks against Iraqi installations housing its personnel.
Pentagon spokesman John Kirby said in a statement: "US military forces earlier this evening conducted defensive precision air strikes against facilities used by Iran-backed militia groups in the Iraq-Syria border region," adding that the attacks were carried out at the direction of US President Joe Biden.
Iraq's Hashed al-Shaabi paramilitary alliance said the US air strikes had "resulted in the martyrdom of a group of heroic fighters" near the Syria border and threatened revenge.
"We will remain the shield defending our beloved nation, and we are fully ready ... to respond and take revenge."
While the US did not provide information on casualties, the Britain-based Syrian Observatory for Human Rights said that "at least five Iran-backed Iraqi militia fighters were killed and several others were wounded in an attack by US warplanes" on the Syrian side of the frontier.
The war monitor, which relies on a network of sources inside Syria to collect information, said that military positions were among the targets hit.
Syria's state-run SANA news agency said one child had been killed and that at least three other people were wounded.
Mr Kirby said that the targets - two in Syria, one in Iraq - were selected because "these facilities are utilised by Iran-backed militias that are engaged in unmanned aerial vehicle (UAV) attacks against US personnel and facilities in Iraq".
Kataeb Hezbollah and Kataeb Sayyid al-Shuhada, two hardline Iraqi military factions with close ties to Tehran, were among the "several Iran-backed militia groups" that had used the targeted facilities.
Since the start of the year there have been more than 40 attacks against US interests in Iraq, where 2,500 American troops are deployed as part of an international coalition to fight the jihadist Islamic State group.
The vast majority have been bombs against logistics convoys, while 14 were rocket attacks, some of them claimed by pro-Iran factions that aim to pressure Washington into withdrawing all US troops.
Mr Kirby said: "Given the ongoing series of attacks by Iran-backed groups targeting US interests in Iraq, the president directed further military action to disrupt and deter such attacks.
"Specifically, the US strikes targeted operational and weapons storage facilities at two locations in Syria and one location in Iraq, both of which lie close to the border between those countries."
The move marks the second such US attack on Iran-backed militia in Syria since President Biden took office, with the first having occurred in February with a strike on facilities in the country's east that left more than 20 fighters dead, according to the observatory.