U.S. aircraft include B-1 bombers, F-16s, F-18s and Predator drones, with F-18s flying missions off the USS George H.W. Bush in the Persian Gulf. IS positions near Raqqa, the Islamic State stronghold in northern Syria, were struck by Tomahawk missiles.
US President Barack Obama is expected to make an official statement with regard to the operation.
Note that, U.S. air strikes have been limited to specific missions in northern Iraq until now.