Iraqi forces are preparing for a fresh push against Islamic State-held Mosul 'within several days' a US general has said, as American soldiers prepare to get closer to the front line than ever before in the two month campaign to take the jihadi stronghold.

Iraqi troops backed by Western advisers, Shia militia fighters and Kurdish forces have retaken some 25% of Iraq's second city since the battle for Mosul began, but the offensive has been bloody and slow. IS has managed to hit back despite tens of thousands of Iraqi soldiers and US air strikes.

The coalition announced an "operational refit" in December that redeployed several thousand police officers from the south of the city to bolster the offensive in the east of the city. US advisers are expected to work closely with the new force in the coming weeks.

"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, told Reuters 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."

Although James said that the risks to US personnel - three of which have been killed in northern Iraq in the last 15 months - was still only classified as 'moderate', the involvement may mark the biggest combat role for US soldiers in Iraq since America officially withdrew from the country in 2011.

"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.

Mosul fell to IS in June 2014 and, along with Raqqa, marks one of two jihadi strongholds in its self-proclaimed 'caliphate' in Iraq and Syria. In the years since since IS took control of the city, conditions are believed to have deteriorated and the entire city rigged with booby traps and road-side bombs.

IS fighters are embedded in the narrow alleys of western Mosul, on the banks of the River Tigris, and believed to be preparing for a fight to the death in one of the cities most populated areas. The Iraqi advance has been hampered by IS using Iraqi civilians - of which some 1.5m are believed to remain in the city - as human shields and encouraging civilian casualties in Western air strikes.

Iraqi Prime Minister Haidar al-Abadi vowed to take back Mosul by the end of 2016, a target that now looks unlikely.