“In recent weeks, ISIS has consolidated its control over a large area in eastern Syria, where it has established a safe haven, from which external operations can be planned,” said a senior U.S. counterterrorism official.

Mosul, Iraq may be the city where ISIS leader Abu Bakr al-Baghdadi publicly proclaimed the existence of a new Islamic state, or “caliphate,” but the capital of his domain is Raqqa, in eastern Syria. The city of 220,000 is where ISIS is at its strongest, and also the place that officials and experts fear could serve as a base of operations for terrorists.

“The Sunni region of Iraq remains an active and chaotic battlefield for the most part, while there are areas of eastern Syria which have become secure redoubts for both ISIS and Jabhat al-Nusrah (the al-Qaeda affiliate fighting in Syria) and serve as home to fixed training camps, bases, and headquarters for these jihadi groups,” said Evan Kohlmann of Flashpoint Intelligence, an NBC News analyst. “It is these locations, such as the city of Raqqa, that present the most serious potential terrorist threat to Western countries including the United States.”