US President Barack Obama said he expects Iraq's second city Mosul to be retaken from the Islamic State group "eventually".

Obama's comments in an interview with CBS News on Monday came on the same day that Defense Secretary Ashton Carter said Washington would send Apache attack helicopters and more troops to Iraq.

"As we see the Iraqis willing to fight and gaining ground, let's make sure that we're providing them more support," Obama said.

"We're not doing the fighting ourselves, but when we provide training, when we provide special forces who are backing them up, when we are gaining intelligence -- working with the coalitions that we have -- what we've seen is that we can continually tighten the noose," he added.

"My expectation is that by the end of the year, we will have created the conditions whereby Mosul will eventually fall."

Mosul, in northern Iraq, was overrun during a jihadist offensive in June 2014 and has become an IS stronghold.

Washington heads an international coalition that is carrying out strikes against IS and also providing training and other assistance to forces fighting the jihadists in both Iraq and neighboring Syria.

Carter, on an unannounced trip to Baghdad, said the Apaches being sent to Iraq will support Iraqi efforts to surround and eventually recapture Mosul.

Iraqi forces have begun preparatory operations in Nineveh province, of which Mosul is the capital, but the actual assault to retake the city is not expected for months, and possibly not until next year.

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