The US-led coalition is "laying the groundwork" for the "isolation" of Raqa, the Islamic State group stronghold in Syria, US Defence Secretary Ashton Carter said Tuesday.

"We have already begun laying the groundwork for our partners to commence the isolation of Raqa," Carter said after meeting coalition defence ministers in Paris to discuss the aftermath of the planned capture of Mosul from IS in Iraq.

"Today we resolved to follow through with that same sense of urgency and focus on enveloping and collapsing ISIL's control of Raqa," he added, using another acronym for IS.

Carter and ministers from 12 other countries attended the talks, which come a week after Iraqi forces backed by Kurdish fighters launched a major operation to retake Mosul, Iraq's second-biggest city.

French President Francois Hollande reiterated warnings about IS fighters in Mosul fleeing across the border to Raqa.

He also urged vigilance over the prospect of foreign militants returning home from the battlefield.