BAGHDAD (Reuters) - The Iraqi army dropped tens of thousands of leaflets over Mosul before dawn on Sunday, warning residents an offensive to recapture the city from Islamic State was in its final stages of preparation, according to a military statement in Baghdad.

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The leaflets carried several messages, one of them assuring the population that advancing army units and air strikes “will not target civilians” and another telling them to avoid known locations of Islamic State militants.

The assault on Mosul, the last city still under control of the ultra-hardline Islamic State in Iraq, could begin this month with the support of a U.S.-led coalition, according to Iraqi government and military officials.

Islamic State fighters are dug in and are expected to fight hard. They have forced civilians to stay in harm’s way during previous battles to defend territory.

Russian President Vladimir Putin said on Sunday he hoped the United States and its allies would do their best to avoid civilian casualties in an attack on Mosul.

Reflecting the authorities’ concerns over a mass exodus that would complicate the offensive, the leaflets told residents “to stay at home and not to believe rumors spread by Daesh” to cause panic, referring to Islamic State by its Arabic acronym.

With a pre-war population of around 2 million, Mosul is around 4-5 times the size of any other city recaptured so far from the militants, who swept through northern Iraq in 2014 and also hold a swathe of Syria.

The U.N. last week said it was bracing for the world’s biggest and most complex humanitarian effort in the battle for the city, which could make up to 1 million people homeless and see civilians used as human shields or even gassed.

“Keep calm and tell your children that it is only a game or thunder before the rain,” a leaflet said. “Women should not scream or shout, to preserve the children’s spirit.”

“If you see an army unit, stay at least 25 meters away and avoid any sudden movements,” another said.

Iraq earlier this month launched a radio station to help Mosul residents stay safe during the offensive.

The radio is broadcasting from Qayyara, a town 60 kilometers (about 40 miles) south of Mosul, where the army is massing forces ahead of the offensive.

Qayyara has also an airfield that will be used as a hub by the U.S.-led coalition to support the offensive in which Kurdish Peshmerga and Sunni tribal fighters are expected to take part.

Iraqi Prime Minister Haider al-Abadi has not yet made it clear whether Iranian-backed Shi’ite paramilitary units will participate in the offensive on the mainly Sunni city.

Local Sunni politicians and regional Sunni-majority states including Turkey and Saudi Arabia have cautioned that letting Shi’ite militias take part in assault could lead to sectarian bloodletting.