Two bombs went off at this Sunni mosque near the Iraqi city of Samarra during Friday prayers. The devices were hidden in an air conditioning unit. At least 15 people have been killed. At least 17 more have been wounded. It wasn't immediately clear who was behind the attack. But Iraq's delicate sectarian balance is under growing strain from the civil war in neighbouring Syria. Both Sunni and Shi'ite Muslims have crossed into Syria from Iraq to join the conflict on both sides.