MAKHMOUR, Iraq (Reuters) - Islamic State is retrenching as Iraqi forces build up for an operation to retake the northern city of Mosul and some local militants desert the group, the commander in charge of the highly anticipated offensive said.

Iraqi security forces ride in vehicles travelling to Mosul to fight against militants of Islamic State at an Iraqi army base in Camp Taji in Baghdad, February 21, 2016. REUTERS/Ahmed Saad

Thousands of Iraqi troops have deployed to the north with heavy weapons in recent weeks, setting up base alongside U.S. forces and troops from Iraq’s Kurdish autonomous region in Makhmour -- a launchpad around 60 kilometers south of Mosul.

Although he described Islamic State as depleted, Nineveh Operations Commander Major General Najm al-Jubbouri said jockeying between the various forces preparing to take part in the battle for Mosul benefited the militants.

“The operation to liberate Nineveh will be done in stages,” Jubbouri said, referring to the province of which Mosul is capital. “Now we are more or less just waiting for the order of the commander in chief to begin the first step.”

Mosul, home to around two million people before it fell to Islamic State during a lightning offensive in 2014, is by far the biggest city ruled by the jihadist group in either Iraq or Syria. An Iraqi offensive to recapture it, backed by air strikes and advisors from a U.S.-led coalition, would be the biggest counterattack ever mounted against the group.

Kurdish and Iraqi military sources say the initial move will be westwards from Makhmour to the town of Qayyara on the Tigris River, which would sever Islamic State’s main artery between Mosul and territory it controls further south and east.

Jubbouri said the timing would hinge on the progress of military operations in the valley of Iraq’s other great river, the Euphrates, where Iraqi forces have been advancing against the militants after routing them from a provincial capital, Ramadi, in December.

The U.S.-led coalition bombing Islamic State hopes Mosul will be recaptured this year, dealing a decisive blow to the militant group. But many question whether the Iraqi army, which partially collapsed when the militants overran a third of the country in June 2014, will be ready in time.

“NO WAY FOR THEM”

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As Iraqi forces push up towards Mosul, Jubbouri said he did not expect to encounter much resistance because people in the villages south of the city were fed up with the militants and are likely to rise up against them.

Already, Islamic State has withdrawn from certain villages, he said: “They have begun to abandon some areas and concentrate their presence near Mosul because they know there is no way for them.”

Citing intelligence, Jubbouri said there were 6,000-8,000 militants in Nineveh province, of which the majority were local Iraqis motivated less by belief in Islamic State’s ultra-hardline creed than the material benefits of siding with the militants.

“For that reason, many have begun to leave the organization and the battlefield. We know the foreign fighters are the ones who will fight fiercely, and also the Iraqis whose hands are stained with the blood of other Iraqis.”

In Mosul itself, Jubbouri expected street fighting and said the main challenge was the presence of more than one million people who would be used as human shields by the militants.

When Islamic State conquered Mosul, many residents welcomed them as liberators from a heavy-handed army, but Jubbouri estimated that between 70-75 percent of the population would now support the security forces.

“The rest are either hesitant or scared, and a part are embroiled (with Islamic State) in a big way.”

Rivalry between different factions who want to take part in the offensive and secure influence in Mosul poses another challenge, Jubbouri said.

The involvement of Shi’ite militias is a subject of controversy because they have been accused of abuses against Sunnis in other areas recaptured from the militants.

Turkey has also deployed troops to a base north of Mosul where they are training a militia formed by the former governor of the province, while Kurdish security forces known as peshmerga will play a support role.

“What is necessary is for all efforts now to be combined,” Jubbouri said. “We must put aside our egos and differences and keep our eyes on the goal: to liberate the city.”