Iraqi officials say U.S. airstrikes have driven some ground commanders of the Sunni radical group Islamic State from northern Iraq across the border into Syria.

Buoyed by a victory over the insurgents at Mosul Dam this week, the Iraqi military renewed efforts to retake Tikrit, a key Sunni city. But the operation appeared to stall on its second day Wednesday.

The U.S. announced a new series of airstrikes that hit Islamic State forces near the dam, while President¬†Barack Obama¬†denounced the group for beheading an American journalist in retribution for the airstrikes. […]