BAGHDAD- Iraq's Popular Mobilisation Forces, a grouping of mostly Iran-backed Shi'ite paramilitaries, said on Monday a U.S. air strike on the Iraqi border with Syria killed 22 of its members and wounded 12 others.

"At 10 pm last night a U.S. plane hit a standing headquarters of the Popular Mobilisation Forces defending the border strip with Syria, using two guided missiles, which led to the martyrdom of 22 fighters," it said in a statement.

It demanded an explanation from the United States.

(Reporting by Ulf Laessing, Ahmed Aboulenein and Ali Abdelaty Editing by Alison Williams and Peter Graff) ((Ulf.Laessing@thomsonreuters.com; Reuters Messaging: follow me on twitter @ulflaessing))