U.S. Secretary of Defense Ash Carter said on Wednesday that the offensive to retake ISIS de-facto capital in Syria, the city of Raqqa, is only weeks away.
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In an interview with NBC News, Carter said it had always been the plan for the Raqqa offensive to follow quickly after the operation to liberate Mosul from ISIS that began 10 days ago. But U.S. forces will not be part of the "occupation" or forces holding Mosul, Carter told NBC.
Carter recently returned from a trip to Iraq where he visited the 5,000 U.S. troops taking part in the campaign to liberate Mosul from ISIS.
"It starts in the next few weeks," he said, referring to the Raqqa offensive, adding that the various forces fighting ISIS – Iraqi, Shi'ite, Kurdish Turkish and others – had the necessary resources to fight in both Mosul and Raqqa.
The top U.S. military commander in Iraq said that Kurdish YPG militia fighters will be a part of the force to isolate the city.
The United States regards the YPG as an ally in its fight against ISIS, but Turkey regards it as a terrorist organization because of its links with Kurdish militants fighting a three-decade insurgency in Turkey.
"The only force that is capable on any near term timeline are the Syrian Democratic Forces, of which the YPG are a significant portion," said Army Lieutenant General Stephen Townsend in a news briefing. "We'll move soon to isolate Raqqa with the forces that are ready to go."
Mosul, a city of 1.5 million people, and the smaller Syrian city of Raqqa are the two pillars of ISIS' self-declared caliphate. Recapturing them would be a pivotal defeat for the ultra-hardline Sunni jihadists.
ISIS itself is expected to morph into a more classic insurgency once it loses its final pockets of territory in Iraq and could lash out abroad with renewed attacks against Western targets, officials caution.
Reuters contributed to this report.