Carter: US forces need to stay in Iraq post-IS

US forces, along with their international coalition partners, will need to remain in Iraq even after the expected defeat of the Islamic State group, American Defence Secretary Ash Carter said Saturday.

Carter said the US and its coalition partners must not stop after completing the current campaign to expel IS from Mosul, adding that the militants are on a path to lasting defeat.

"But there will still be much more to do after that to make sure that, once defeated, ISIL stays defeated," he said, using an alternative acronym for the Islamic State group.

"We'll need to continue to counter foreign fighters trying to escape and ISIL's attempts to relocate or reinvent itself. To do so, not only the United States but our coalition must endure and remain engaged militarily."

"In Iraq in particular, it will be necessary for the coalition to provide sustained assistance and carry on our work to train, equip and support local police, border guards and other forces to hold areas cleared from ISIL."

He did not say how long this continued US military presence might be necessary or how many troops would be required.

At any rate, those decisions are likely to fall to the Trump administration after it takes office in January.

In describing recent Pentagon actions to put a stranglehold on the IS worldwide, Carter said the Obama administration has directed the secretive Joint Special Operations Command to prioritise destroying the militant group's ability to conduct attacks in the West.

Carter said that in his final weeks in office he is focused on ensuring a smooth transition to his successor.

Earlier this week, President-elect Donald Trump announced that he intends to nominate retired Marine Gen. James Mattis to be his defence secretary.

Carter congratulated Mattis, who is a former commander of US Central Command overseeing the wars in Iraq and Afghanistan.

Agencies contributed to this report.