Iraqi forces will resume their push against Islamic State inside Mosul in the coming days, a U.S. battlefield commander said, in a new phase of the two-month-old operation that will see American troops deployed closer to the front line in the city.

The battle for Mosul, involving 100,000 Iraqi troops, members of the Kurdish security forces and Shi'ite militiamen, is the biggest ground operation in Iraq since the U.S.-led invasion of 2003.

The upcoming phase appears likely to give American troops their biggest combat role since they fulfilled President Barack Obama's pledge to withdraw from Iraq in 2011.

Elite Iraqi soldiers have retaken a quarter of Mosul, the jihadists' last major stronghold in Iraq, but their advance has been slow and punishing. They entered a planned "operational refit" this month, the first significant pause of the campaign.

A heavily armoured unit of several thousand federal police was redeployed from the southern outskirts two weeks ago to reinforce the eastern front after army units advised by the Americans suffered heavy losses in an Islamic State counter-attack.

U.S. advisers, part of an international coalition that has conducted thousands of air strikes and trained tens of thousands of Iraqi ground troops, will work directly with those forces and an elite Interior Ministry strike force.

"Right now we're staging really for the next phase of the attack as we start the penetration into the interior of east Mosul," Lieutenant Colonel Stuart James, commander of a combat arms battalion assisting Iraqi security forces on the southeastern front, said in a Reuters interview late on Sunday.

"So right now, positioning forces and positioning men and equipment into the interior of east Mosul... it's going to happen in the next several days."

That will put U.S. troops inside of Mosul proper and at greater risk, though James said the danger level was still characterised as "moderate". Three U.S. servicemen have been killed in northern Iraq in the past 15 months.

James, speaking from an austere outpost east of Mosul where several hundred U.S. troops are stationed, said the pace of the upcoming phase on the eastern side would depend on resistance from Islamic State, also known as ISIS, ISIL or Daesh.

"If we achieve great success on the first day and we gain momentum, then it may go very quickly. If Daesh fights very hard the first day and we run into a roadblock and we have to go back and go OK that was not the correct point of penetration, it may take longer," he said.

INTEGRATION

Further integration with the Iraqi troops - to what commanders described as an unprecedented level for conventional U.S. forces - will help synchronize surveillance, air support and force movement, according to James.

"It increases our situational understanding. The man on the ground knows what's going on best," he said. "It's just better when they're on the ground talking to each other and saying, 'Hey, have you looked at that area over there? That's decisive terrain. Have you thought about putting forces there?'"

Mosul, the largest city held by Islamic State anywhere across its once vast territorial holdings in Iraq and neighbouring Syria, has been held by the group since its fighters drove the U.S.-trained army out in June 2014.

Its fall would probably end Islamic State's ambition to rule over millions of people in a self-styled caliphate, but the fighters could still mount a traditional insurgency in Iraq, and plot or inspire attacks on the West.

A multi-ethnic city where up to 1.5 million people of a pre-war population of around 2 million are still thought to be living, Mosul is divided roughly in half by the Tigris River. The western section, which Iraqi forces have yet to penetrate, has built-up markets and ancient narrow alleyways which will complicate future advances.

Iraqi Prime Minister Haider al-Abadi had said he would win Mosul back by the end of this year, a deadline now certain to be missed. His commanders say their advance was held up by the need to protect civilians, fewer of whom fled than initially expected.

Inclement weather has repeatedly delayed ground advances which rely heavily on aerial surveillance and air strikes.