Preparations for the assault on Iraq's second city have been going on for months, while the UN warns it could spark the world's biggest humanitarian crisis.

The UN also fears that IS will use civilians as human shields once the battle begins.

Iraqi special forces are being deployed around Mosul's suburbs, and the push on the city may come from at least three different directions, with other army units deployed south of the city.

Mosul was seized by IS - or Daesh - in June 2014 after Iraq forces fled, and remains the last major city they hold.

On Saturday night, the Iraqi air force dropped thousands of leaflets on Mosul warning residents that the battle was imminent.

"It's victory time," said one of the leaflets. "Time to celebrate a clean Iraq without 'Daesh' or any dark belief."

Another leaflet said: "We are preparing to take action to free Mosul and regain security and stability in the region."

The Islamic State news agency Amaq has released pictures it says shows ongoing fighting in the Refaq and al Masarif neighbourhoods of Mosul.

The offensive will be a joint operation by the Iraqi military, Kurdish Peshmerga and a Shia paramilitary force.

It is thought that around 30,000 troops will be needed to retake the city.

But there have been fears that ethnic and religious rivalries could undermine a cohesive military campaign.

Worst case scenario

Masoud Barzani, the President of the Kurdistan Regional Government, tweeted: "The time has come to begin the liberation of Mosul."

In a statement, he added: "The preparation for the operation to liberate Mosul (has) been completed and (has) paved the way to begin the Mosul operation." 

The UN refugee agency, the UNHCR, fears more than one million people could flee Mosul once the military operation gets under way.

It says it would need $1bn to house and feed that many people - and so far has just $230m.

Lise Grande, the UN Humanitarian Coordinator for Iraq, said: "The worst case scenario in Mosul would look something like this: you would have mass expulsion of hundreds of thousands of people. 

"You would have hundreds of thousands of people who are held as human shields.

"You would have a chemical attack that would put tens of thousands, hundreds of thousands or maybe even more at grave risk.

"If all that were to happen at the same time it would be catastrophic," 

The UNHCR has so far built six camps which could house 50,000 people, and there are plans to build 11 more.

Three million people have already been displaced in Iraq because of Islamic State.