International media outlets report that the United States on Tuesday made its strongest indication yet that the battle to take Raqqa, the stronghold of the Islamic State (IS) terrorist group in Syria, could be fast approaching, saying it would “overlap” with an already unfolding assault in Iraq to seize the city of Mosul.

U.S. Defense Secretary Ash Carter, speaking to reporters after a gathering of 13 countries in the U.S.-led coalition in Paris, did not disclose the timing of the Raqqa campaign but said preparations were on track.

“Yes, there will be overlap (in the Mosul and Raqqa campaigns) and that's part of our plan and we are prepared for that,” Carter was cited as saying by Reuters.

Mosul, a city of 1.5 million people, and the smaller Syrian city of Raqqa are the two pillars of Islamic State's self-declared caliphate, and recapturing them would be a pivotal defeat for the ultra-hardline Sunni jihadists.

A senior U.S. military official, speaking to reporters traveling with Carter, suggested the Raqqa kick-off would likely follow some additional successes in Mosul.

He said the operation in Raqqa would be carried out by “capable and motivated local forces that we identify and enable.”

The official also said military planners would seek to avoid overstretching U.S.-led coalition assistance, which includes air strikes.

ABC reports that French President Francois Hollande, addressing a meeting of anti-IS coalition partners in Paris, said the offensive on Mosul could trigger an outflow of foreign fighters — a concern for European nations wary of attacks by IS militants returning from Iraq and Syria.

“In these columns of people leaving Mosul will be hiding terrorists who will try to go further, to Raqqa in particular," Hollande said, calling for greater intelligence sharing.

“Raqqa will be the last bastion of Daesh if Mosul falls,” Hollande said, using an Arabic acronym for the IS group.  “Daesh must be eliminated everywhere.  Any stronghold represents a threat.”

His comments came as Iraqi government forces pushed closer to Mosul, the last IS stronghold in Iraq.

Fighting continued on October 25 in a belt of villages and towns to the north, east, and south of Mosul, Iraq's second-largest city.

U.S. officials acknowledge the campaign to retake control of Mosul could take weeks, if not months, giving military planners time if they want both campaigns to run concurrently at some point.