Kurdish forces pause advance 30km east of ISIL-held city while government forces press on with next stage of operation.

Armed forces closing in on Mosul say they have secured about 20 villages on the outskirts of the city in the first 24 hours of an operation to retake the last major stronghold in Iraq of the Islamic State of Iraq and the Levant (ISIL) group.

With air support from the US-led coalition, government and Kurdish forces edged closer to the city on Monday as smoke darkened the blue sky above one ISIL position, apparently from oil fires ignited to hamper the incursion and make it harder to land air strikes.

Kurdish forces announced on Tuesday that they were pausing their advance 30km east of Mosul as the Iraqi army presses ahead with the next stage of the operation.

Colonel Khather Sheikhan of the Kurdish Peshmerga said his troops had achieved their objectives and “are just holding our positions” in the Khazir area.

The pause comes after a day of intense fighting involving air strikes, heavy artillery and ISIL car bombs.

‘Decisive moment’

With a population of 1.5 million, Mosul is the largest city under ISIL’s control.

The recapture of the city would be a “decisive moment” in the war against ISIL, according to Ash Carter, the US defence secretary.

But the urban battle ahead – in a city four to five times larger than other towns seized by ISIL – poses not only a military challenge, but also a humanitarian one, with the United Nations warning of an exodus of up to a million people.

“Away from the military challenges, there is a humanitarian crisis on a massive scale. The UN says up to one million people could flee the city of Mosul once the fighting intensifies, and they would be pouring into a system that is already overstretched with hundreds of thousands,” said Al Jazeera’s Stefanie Dekker, reporting from the Khazir front line.

About 4,000 to 8,000 fighters are thought to be dug into Mosul while the forces assembled to drive them out are estimated at 30,000, including the Iraqi army, Kurdish Peshmerga, Sunni tribal fighters and the Popular Mobilisation Forces – Shia militias groups that now have official status from Baghdad.

More than 5,000 US soldiers are also deployed in support missions, as are troops from France, Britain, Canada and other western nations.

The Iraqi army is attacking Mosul on the southern and southeastern fronts, while the Peshmerga forces are carrying out their operation to the east.

IN PICTURES: Knocking on Mosul’s door

The Peshmerga, who are also deployed north and northwest of the city, said they secured “a significant stretch” of the 80km road between Erbil, their capital, and Mosul, about an hour’s drive to the west.

The UN refugee agency said it had built five camps to house 45,000 people and plans to have an additional six constructed in the coming weeks with a capacity for 120,000, although that would still not be enough to cope if the exodus is as big as feared.

The battle has also raised fears of sectarian and ethnic revenge killings in territory retaken from ISIL.

Amnesty International urged Iraqi authorities on Tuesday to keep Shia militia groups away from Mosul, a city whose population is largely Sunni.

The rights group said the government in Baghdad would bear responsibility for the actions of the militias, known collectively as the Popular Mobilisation Forces, which are officially considered to be part of the country’s armed forces.

“There can be no justification for extrajudicial executions, enforced disappearances, torture or arbitrary detention,” said Amnesty’s Philip Luther.