The US in response to drone attacks on US forces on Sunday carried out air strikes against Iran-backed militia groups in the Iraq-Syria border region.
According to a statement by the Pentagon, the strikes hit “operational and weapons storage facilities.”
President Joe Biden would “act to protect US personnel”, it added.
The Pentagon has not provided any information on fatalities,but a monitoring group reported that five militia fighters were killed in Syria.
The UK-based Syrian Observatory for Human Rights said several more fighters were wounded “in an attack by US warplanes”.
However, Syrian state news agency Sana reported that a child had died and at least three other people were wounded.
This is the second round of air strikes against Iran-backed militias since President Biden took over power in January.
US forces based in Iraq have been targeted several times in recent months by drone attacks. Iran has denied any involvement.
About 2,500 US troops are based in the country as part of an international coalition fighting the jihadist Islamic State (IS) group.
According to the Pentagon, the “defensive precision air strikes” hit two targets in Syria and one in Iraq. It said several Iran-backed militia groups, including Kataib Hezbollah and Kataib Sayyid al-Shuhada, used these facilities.
Since 2009, the US has regarded Kataib Hezbollah as a terrorist organisation, accusing it of threatening the peace and stability of Iraq.