Many ISIS commanders have fled the besieged cities of Mosul, Iraq, and Raqqa, Syria, in recent months for Mayadeen, US officials and Syrian activists say.
Although the US-led coalition is aware of the migration and has launched airstrikes there, its military campaign remains focused on ousting ISIS from its two centers of operation, Mosul and Raqqa.
Militants may already be communicating, coordinating and directing attacks from Mayadeen, about 190 kilometers southeast of Raqqa, at a time when foreign fighters are returning to home countries in Europe, the Middle East, North Africa and Asia and conducting attacks.
Command and control of the overall organization is now in the city, said several U.S. officials, who weren't authorized to speak publicly on the matter and demanded anonymity.
US defense officials dismiss suggestions the coalition hasn't acted forcefully enough to prevent IS leaders from moving headquarters and regrouping. But they agree the preponderance of their operations are in Mosul and Raqqa, ABC news reported.