BRUSSELS (AFP) - The offensive to drive the ISIS militant group from its Raqqa stronghold in Syria will begin in the next few weeks, top US and British defence officials said on Wednesday (Oct 26).

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 Islamic State in Iraq and Syria control.

"It starts in the next few weeks," Carter said, referring to the timeline for the Raqqa offensive in an interview with NBC news before arriving in Brussels for a Nato meeting.

"That has long been our plan and we will be capable of resourcing both," Carter told the US network.

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

Arriving for the two-day Nato defence ministers meeting in the Belgian capital, Fallon said: "We hope a similar operation will begin towards Raqqa in the next few weeks."

The United States leads a 60-nation anti-ISIS 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 ISIS leaders declared their "caliphate" - would leave Raqqa the last major city still under the group's control.

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