The US will deploy additional forces to Iraq and make Apache attack helicopters available to support the country's troops, US Secretary of Defence Ashton Carter said on Monday in Baghdad.

"We are going to bring in additional forces," Carter said, adding that the Apaches will support Iraqi efforts to surround and eventually recapture second city Mosul from the Islamic State jihadist group.

A senior US defence official said around 200 additional American military personnel would be deployed, pushing the total figure past 4,000.

The US 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 neighbouring Syria.

It has been bombing IS since August 2014, but most of the strikes are carried out by planes or drones.

Carter arrived in Baghdad from the United Arab Emirates, his first stop on a Gulf tour during which he will seek to shore up support for Iraq.

IS overran large areas north and west of Baghdad in 2014, but Iraqi forces have since regained significant ground from the jihadists.

While most American forces in Iraq play advisory and support roles, Washington has also sent in special forces to carry out raids against IS, and US Marines have deployed to provide artillery support for Iraqi troops.

US President Barack Obama repeatedly pledged that there would be no "boots on the ground" to combat IS, but US forces are engaged in combat with the jihadists and two American military personnel have already been killed.

An American Marine was killed in a rocket attack in northern Iraq last month, and a US special forces soldier wounded during a raid last year later died.

Both the raid and the attack on the Marine artillery position only came to light after American personnel were killed.

US-led forces invaded Iraq in 2003, overthrowing dictator Saddam Hussein and unleashing an insurgency and years of brutal sectarian violence.

Rampant violence was eventually brought under control, and American forces withdrew at the end of 2011 after talks on a residual troop presence broke down over Washington's insistence that they have parliament-approved immunity.

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