PARIS • As thousands of Iraqi troops slowly encircle the Iraqi city of Mosul, US Defence Secretary Ashton Carter met key coalition members in France this week, suggesting the battle for ISIS' de facto capital in northern Syria could soon begin.

"Yes, there will be overlap (in the Mosul and Raqqa campaigns) and that's part of our plan and we are prepared for that," said Mr Carter, after a gathering of 13 countries in the US-led coalition.

He added the coalition is helping to build the local force, including a contingent of Arab fighters, that will be used to enter Raqqa.

A US military official, speaking on condition of anonymity, said the allies hope to grow the current contingent of US-backed fighters in Syria by a "couple hundred" before the force goes into Raqqa, hopefully using past military successes to attract recruits.

Earlier in the year, US military officials suggested that the campaigns to retake Mosul and Raqqa could happen simultaneously.

But with the Islamic State in Iraq and Syria (ISIS) stronghold of Mosul three times the size of Raqqa, Iraqi forces need to make considerable gains in Mosul before the coalition can muster enough resources to support both operations, the military official said.

While the fight for Raqqa could be months away, more than 30,000 Iraqi troops under various flags are slowly inching towards the centre of Mosul in the campaign that began earlier this month to retake the northern Iraqi city.

ISIS fighters yesterday kept up their fierce defence of the southern approaches to Mosul, which has held up Iraqi troops on the southern front and forced an elite army unit east of the city to put its more rapid advance on hold.

But Mosul residents said that the militants were shaving their beards and changing hideouts to be closer to escape routes to Syria, probably in preparation "to escape the city".

French President Francois Hollande, addressing Tuesday's coalition meeting, said the offensive on Mosul could trigger an outflow of foreign fighters - a concern for European nations wary of attacks by ISIS militants returning from Iraq and Syria.

"We must also be very vigilant towards the return of foreign fighters," said Mr Hollande.

The gathering came just weeks before the first anniversary of ISIS attacks in Paris on Nov 13 last year that killed 130 people.

It was followed by other incidents in the US and Europe perpetrated or inspired by the group.

WASHINGTON POST, REUTERS, AGENCE FRANCE-PRESSE