The movements are seen by U.S. ­counterterrorism analysts as a worrisome indication of the expanding appeal of a group known as the Islamic State that has overwhelmed military forces in the region and may now see itself in direct conflict with the United States.

“Small groups from a number of al-Qaeda affiliates have defected to ISIS,” as the group is also known, said a U.S. official with access to classified intelligence assessments. “And this problem will probably become more acute as ISIS continues to rack up victories.”

The influx has strengthened an organization already regarded as a menacing force in the Middle East, one that has toppled a series of Iraqi cities by launching assaults so quickly and in so many directions that security forces caught in the group’s path have so far been unable to respond with anything but retreat.

U.S. officials attribute the Islamic State’s rapid emergence to factors both psychological and tactical.