Kerry suffered a likely leg injury, but did not lose consciousness, State Department spokesman John Kirby said. The top American diplomat is in stable condition.

The executions came just hours after Islamic State militants released a video that purportedly showed the captured pilot being burned alive in a cage.

Sen. John McCain says a pardon for Jack Johnson would be a message to the world that America is determined to "live up to the noble ideals of freedom, opportunity, and equal justice for all."

The video, which could not be independently verified, was released by the Islamic State's media arm, according to the IntelCenter, which monitors extremist websites.