As the battle in Mosul rages on, defense heads of the coalition against ISIL gathered in Paris on Tuesday. US Secretary of Defense Ashton Carter says the coalition is set to begin an operation to retake the city of Raqqa, Mosul's counterpart ISIL stronghold in Syria.

The United States expects the campaigns against Islamic State in Iraq's Mosul and Syria's Raqqa to overlap.

And a push to start retaking the group's de facto capital in Syria may not be very far off. That was a message that US Defense Secretary Ashton Carter sent during the anti-ISIL coalition meeting in Paris.

"We've already begun laying the groundwork with our partners to commence the isolation of Raqqa. As we meet here, we're helping to generate the local forces that will do so. This is one of our campaign plan's core objectives, destroying ISIL's parent tumor in Iraq and Syria. And we're on track to do just that," Carter said.

But question is - which ground force will work with the US-led coalition to retake the ISIL stronghold in Syria?

That is what has Russia and its ally the Syrian government concerned, especially when it comes to those who backed by western powers while considered as terrorists by Russia's coalition.

"Our conclusion remains the same. The most important condition is separating the so-called moderate Syrian opposition from groups like ISIL. Our American partners promised us to do it 8 months ago. Nothing has happened yet," Russian FM Sergey Lavrov said.

The US and its allies did not reveal specific measures on a possible operation in Raqqa.  But the focus of the anti-ISIL campaign may be shifting to Syria.