The strikes, ordered by President Joe Biden on Sunday, were made in response to drone attacks by the militia against US personnel and facilities in Iraq. Operational and weapons storage facilities at two locations in Syria and one location in Iraq were targeted, the US military said in a statement.

       It is the second time Mr Biden has ordered retalitory strikes against Iran-backed militia since taking office in January.

       In February, strikes were ordered in response to rocket attacks from Iraq, targeting a location in Syria.

       "As demonstrated by this evening's strikes, President Biden has been clear that he will act to protect U.S. personnel," the Pentagon said in a statement.

       It is not known whether anybody has been killed or injured.

       Iran-backed militia, including Kataib Hezbollah and Kataib Sayyid al-Shuhada, used the facilities, the Pentagon said.