The offensive to drive the Islamic State group from its Raqa stronghold in Syria will begin in the next few weeks, top US and British defence officials said Wednesday.

US Defence Secretary Ashton Carter and his British counterpart Michael Fallon made the predictions nearly 10 days into a US-backed Iraqi offensive on Mosul, the last major Iraqi city under IS control.

"It will be within weeks, not many weeks," Carter told reporters at a two-day NATO defence ministers meeting in Brussels.

During a visit Sunday to Iraq's autonomous region of Kurdistan to review the Mosul offensive, Carter said an operation to isolate IS in Raqa should begin in conjunction with the assault on its Iraqi bastion.

Arriving for the defence ministers meeting, Fallon said: "We hope a similar operation will begin towards Raqa in the next few weeks."

The United States leads a 60-nation anti-IS coalition that has provided key support for the Iraqi army offensive launched last week.

It comes in the form of thousands of air strikes, training for Iraqi forces and advisers on the ground.

The loss of Mosul -- where IS leaders declared their "caliphate" -- would leave Raqa the last major city still under the group's control.

Carter said the idea of simultaneous operations against Mosul and Raqa "has been part of our planning for quite a while."

Later Wednesday, he said the United States would likely work with key NATO ally Turkey to retake Raqa when asked about tensions between Baghdad and Ankara over its role in the attack on Mosul.

"We already are working extensively with the Turkish military in Syria" and this had produced "significant" results, including the seizure of the "very important city" of Dabiq.

Earlier this month, Ankara-backed rebels seized the emblematic northern town of Dabiq from the extremist group, having already taken Al-Rai.

"So we are looking for other opportunities including further within Syria, to include Raqa. That's been part of our discussions," Carter added.

Turkey's positioning of troops in northern Iraq and its insistence that it has a role in the Mosul offensive have led to sharp exchanges with the Iraqi government, creating a headache for Washington as it tries to keep both of its allies on-side.

Iraq on Monday insisted that Turkey was not participating in the Mosul attack, rejecting Turkish claims that it was.