Federal police forces launch a rocket during clashes with ISIS militants in south of Mosul.

Iraq’s battle to reclaim Mosul from the Islamic State of Iraq and Syria is progressing despite “stiff” resistance from resilient and creative fighters, but a coming offensive to oust them from their main Syria stronghold at Raqqa poses tougher political challenges and could take longer, the US commander of anti-ISIS coalition forces said on Wednesday.

Army Lt. Gen. Stephen Townsend also said American intelligence has detected indistinct signs of ISIS plotting against the West from Raqqa, adding urgency to coalition plans to encircle and eventually assault the city.

“We know they’re up to something, and it’s an external plot,” the general told reporters at the Pentagon, speaking from his headquarters in Baghdad. “We don’t know exactly where” they intend to strike, he added. “We don’t know exactly when.”

“We know this plot-and-planning is emanating from Raqqa. We think we’ve got to get to Raqqa pretty soon.” He declined to be more precise about a timeline but said there is “a sense of urgency” due to the uncertainty of the threat.

As ISIS’ two main hubs, Mosul and Raqqa have long loomed as potentially decisive battles. The group has lost considerable territory elsewhere in both Iraq and Syria over the past year, and the U.S. and its coalition partners see collapsing their last major strongholds as a key to extinguishing extremist claims to a caliphate.

Townsend and other coalition leaders are struggling with the timing for Raqqa, not only because of the demands of the large Iraqi-led Mosul operation but also because the political and military landscape in Syria is more complicated. Townsend said more Syrian opposition fighters need to be recruited, trained and equipped for the Raqqa battle, but he and other officials have said in recent days that the Mosul and Raqqa operations will overlap.

“I think it will be within weeks,” Defense Secretary Ash Carter said in Brussels, adding, “Not many weeks.”

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