Carter said Washington is willing to do more to support Iraqi security forces as they battle Islamic State and eventually set their sights on Mosul, Iraq's second-largest city that has been under the jihadists' control since June 2014.

Powerful Iranian-backed Shi'ite militia groups who are aligned with the Shi'ite Abadi's government against Islamic State also oppose a greater US presence.

Islamic State (IS) is an ultra-hardline Sunni Muslim militant group that controls swathes of territory in Syria and Iraq and has a presence in parts of other Arab countries such as Egypt and Libya.

Washington has sought to step up its military campaign against the jihadists, who have killed thousands of non-Sunni people in Iraq and Syria and claimed responsibility for attacks in the West, including shootings and suicide bombings in Paris on Nov 13 that killed 130 people.

Carter also told Congress this month that the United States is willing to deploy advisers and attack helicopters if requested by Iraq to help it retake Ramadi, west of Baghdad. The United States has around 3,500 troops in Iraq now.

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