(WN)The Iraqi Air Force dropped millions of leaflets over western Mosul late Saturday warning residents of an offensive by ground forces on the ISIS-held part of the city, which has so far been targeted only by airstrikes. Iraqi forces have had control of the eastern part of the city, which is divided by the Tigris River, since January.

The leaflets say Iraqi forces are making advancements to the western side and "provide guidance and recommendations" for citizens ahead of the offensive, according to a statement from the Iraqi Joint Operations Command (JOC).

The leaflets also warn ISIS members to "lay down their weapons and surrender before they face their inevitable fate at the hands of our heroic forces," the statement said.

As many as 800,000 civilians live in western Mosul, according to the United Nations. It said UN humanitarian agencies in Iraq are preparing to assist civilians caught in the fighting. Iraq's prime minister on Sunday announced an offensive to seize control of western Mosul from the terror group ISIS.

Iraqi Prime Minister Haider al-Abadi said on state television Sunday morning that the new push has begun. Hours earlier, Iraq's military said planes dropped leaflets into the area, urging those who joined ISIS to surrender, and warning that the military would move into the western coast of the city, which straddles the Tigris River.

"I announce today the start of military offensive to liberate the western coast of Mosul," al-Abadi said. He added, "our mission is to liberate people before land."

ISIS seized control of the northern city in 2014, and Iraqi forces entered the city for the first time since then in November. Mosul has been called ISIS' capital in Iraq, although its stronghold is in the Syrian city of Raqqa.

The Iraqi military, backed by the U.S. and other Western allies, have effectively surrounded the city. There are an estimated 750,000 civilians in the western section of the city, according to the United Nations and other groups.

The United Nations Office for the Coordination of Humanitarian Affairs said Saturday that the situation for civilians trapped there is dire, as a road to Syria has been cut off and food and fuel supplies have dwindled.

"People, right now, are in trouble. We are hearing reports of parents struggling to feed their children and to heat their homes," Lise Grande, the Humanitarian Coordinator for Iraq, said in a statement.

The agency was readying to prepare to distribute supplies, warning that tens of thousands of people may flee or be forced to leave the city during the military campaign.

"We don't know what will happen during the military campaign but we have to be ready for all scenarios. Tens of thousands of people may flee or be forced to leave the city. Hundreds of thousands of civilians might be trapped maybe for weeks, maybe for months," Grande said.