Albuquerque (United States) (AFP) - The United States will send about 600 extra troops to Iraq to train local forces for an offensive on the Islamic State group stronghold of Mosul, Defense Secretary Ashton Carter said Wednesday.

IS seized Mosul along with other areas in June 2014, but the country's forces have since regained significant ground from the jihadists and are readying for a drive to retake Iraq's second-largest city.

"These (US) forces will be primarily to enable Iraqi security forces, and also peshmerga, in the operations to isolate and collapse ISIL's control over Mosul," Carter told reporters on a work trip to New Mexico, using an IS acronym. Peshmerga are Kurdish fighters.

"Also to protect and expand Iraqi security forces' gains elsewhere in Iraq," he added.

The US forces will head to Qayyarah, a strategically vital air base 40 miles (65 kilometers) south of Mosul that will help funnel supplies and troops toward the city, as well as to other locations including the joint Iraqi-US Al Asad air base.

They notably will beef up flight capabilities at Al Asad for night operations and operations in low visibility, such as poor weather.

A US-led coalition is carrying out air strikes against IS in Iraq, and Washington has already authorized the deployment of more than 4,600 military personnel to the country.

Most are in advisory or training roles, working with Iraqi and peshmerga forces, but some American troops have fought IS on the ground, and three members of the US military have been killed by the jihadists in Iraq.

Earlier Wednesday, Prime Minister Haider al-Abadi's office indicated it had requested "a final increase in the number of American trainers and advisers" to support Iraqi troops in the northern city.

The statement from Abadi's office noted that US forces are helping Iraq in its battle against the jihadists. But their presence remains extremely politically sensitive due to the nine-year war the United States fought in the country.

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The statement said the number of trainers and advisers would start to be reduced as soon as Mosul is retaken from IS, and also asserted that no American troops had fought alongside Iraqi troops.

In reality, American special forces have fought IS alongside Iraqi Kurdish forces on several occasions that have been made public, and likely in other operations that have not come to light.

Carter said the additional US forces would be tasked with training Iraqis, gathering intelligence and providing logistical support for the Mosul push.

"But I need to make clear... American forces combating ISIL in Iraq are in harm's way... no one should be in any doubt about that," Carter said.

How long they will stay is up to the Iraqis, Carter insisted.

"We are certainly to continue to help the Iraqi security forces in whatever measure and manner they wish to consolidate the control over their country after they've recaptured this last major ISIL center," he said.

According to Pentagon spokesman Navy Captain Jeff Davis, the new deployment is for 615 troops. That would bring total US forces in Iraq to 5,262.

The actual number would be higher than that because the tally does not include certain assignments.

The troops "will move very soon," Davis said.

- A million displaced? -

Carter expects the Mosul offensive to begin in the coming weeks, but stressed the decision was an Iraqi one.

IS has had two years to reinforce its defenses in Mosul, and observers are expecting a difficult fight amid a civilian population.

"We are prepared for whatever happens there," Carter said.

The United Nations warned that military operations there could cause as many as a million people to be displaced.

Last week, US President Barack Obama said US-backed Iraqi troops could be in a position "fairly rapidly" to liberate Mosul, though he warned "this is going to be hard, this is going to be challenging."

Separately, the US military concluded Tuesday that a rocket fired this month at the Qayyarah air base, which houses hundreds of US troops, contained no mustard agent, as initially suspected.

In neighboring Syria, hundreds of US forces are deployed alongside Kurdish and rebel fighters to battle IS, which is also facing air raids by the international coalition.

The Pentagon has expressed concern IS fighters could use mustard gas to defend Mosul.

Even after Mosul is retaken, the war against IS will be far from over.

The jihadists are likely to revert to insurgent tactics, such as bombings of civilians and hit-and-run attacks on security forces, following the demise of their so-called "state" in Iraq.