More than three months of U.S. airstrikes in Syria have failed to prevent Islamic State militants from expanding their control in that country, according to U.S. and independent assessments, raising new concerns about President Barack Obama’s military strategy in the Middle East.

While U.S. bombing runs and missile strikes have put Islamic State forces on the defensive in Iraq, they haven’t had the same kind of impact in Syria. Instead, jihadist fighters have enlarged their hold in Syria since the U.S. started hitting the group’s strongholds there in September, according to the new estimates.

Islamic State’s progress in Syria is partly the result of the U.S. decision to focus its military efforts on Iraq, where the militant group has seized major parts of the country and declared them part of a new Islamic caliphate. The U.S.-led military effort has pushed the forces out of some key battlegrounds in Iraq. […]