(CNN) The Iraqi Air Force dropped thousands of leaflets over Mosul on Saturday night to warn residents of a looming offensive on the ISIS-held city.

"It's victory time," blares a headline on the newspaper-style leaflets, quoting Iraqi President Haider al-Abadi in remarks directed at Mosul's residents. "Time to celebrate a clean Iraq without 'Daesh' (ISIS) or any dark belief."

Another headline reads, "We are preparing to take action to free Mosul and regain security and stability in the region."

The four-page leaflets tell residents that coalition forces are making advances on Iraq's second-largest city and that no one should panic. Residents are asked to stay away from certain parts of the city, avoid ISIS positions, remain in their homes and seal their windows and doors. A phone number for Mosul residents to report ISIS activity is included.

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Iraq's Joint Military Operations said in a statement that thousands of leaflets rained down on the city center of Mosul on Saturday night.

"The newspapers have important information, to update them with the latest facts and victories," the statement said.

Residents have had limited access to the outside world since Mosul fell into the hands of the terrorist group in June 2014. ISIS imposed extreme restrictions on travel in and out of the city, and banned satellite dishes in efforts to limit access to the outside world.

The warning to residents came hours after ISIS started telling wounded fighters in Mosul that they can go to the group's power base in Syria as Iraqi forces and others prepare to retake the key city, a source inside Mosul told CNN.

Preparing for battle

The Iraqi military, Kurdish Peshmerga and Hashd al Shabi, or People's Mobilization Units (PMUs, paramilitary forces) are getting ready for a battle expected to take place in the air and on the ground.

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ISIS is releasing some low-level prisoners, the source said, such as those jailed for their beards, cigarettes or clothing offenses. Wounded ISIS fighters have been told to go to Raqqa, Syria, the group's de facto capital, the source said.

The source inside Mosul said a tunnel network is growing, including one big enough for motorbikes, from the outskirts of the city to the nearby village of Hamdania.

A man imprisoned by ISIS in Mosul for two months said he was forced to help dig more than 12 tunnels on the eastern side of the city.

Waiting for zero hour

Among the groups joining to fight ISIS in Mosul are the Shia-led Hashd Al-Shaabi, or Popular Mobilization Unit.

Some 9,000 Sunni fighters are part of the PMU, said Ahmed al-Assadi, a PMU spokesman. He said the PMU forces also include Christians, Yazidis and Turkmen.

"The troops of the Popular Mobilization Forces are waiting for the launch of the zero hour to participate in the battle to liberate Mosul," Assadi said.

Iraqi Prime Minister Haider al-Abadi meets PMU leaders Saturday.

Members of the PMU met Saturday with the Iraqi prime minister to discuss support of the Iraqi Security Forces on the ground in the Mosul operations. They also talked about fighting ISIS in Hawija, about 100 miles south of Mosul.

Kurdish Peshmerga forces are getting ready for the battle as well. Masoud Barzani, President of the Kurdistan Regional Government, tweeted Saturday, "The time has come to begin the liberation of Mosul."

The time has come to begin the liberation of Mosul https://t.co/vyvP1WzJEH — Masoud Barzani (@masoud_barzani) October 15, 2016

"The preparation for the operation to liberate Mosul (has) been completed and (has) paved the way to begin the Mosul operation," Barzani said in a statement.

Peshmerga forces and the Iraqi military have already made plans for the future of the city, he said.

"Baghdad and Irbil (the capital of Iraqi Kurdistan) have also agreed to establish a joint higher political committee whose task would be to supervise the affairs of Mosul after the liberation."

ISIS executes some who flee

Witnesses inside Mosul said they saw six buses loaded with ISIS members leaving the city early Saturday. Women and children were seen inside the buses.

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Later Saturday, 14 members of ISIS were executed after trying to flee with their families from Mosul to Raqqa, the witnesses said.

Sources said they believe those executed could have been the same people seen leaving by buses earlier Saturday.

On Friday, ISIS set fire to trenches filled with crude oil outside northeastern Mosul, according to military sources. Black smoke has intensified Saturday.

UN refugee agency prepares, too

The UN refugee agency said more than one million people could be fleeing Mosul once the military operation gets underway. They'll need assistance and shelter, said UN High Commissioner for Refugees Filippo Grandi, who is on a four-day visit to Iraq.

"We are mobilizing significant resources to set up camps to accommodate displaced people in safety, and to provide relief items to families who may be on the move," he said.

Suicide bombing in Baghdad

As forces push toward Mosul, Baghdad has become a target of ISIS suicide attacks.

Saturday, at least 34 people were killed and 35 others wounded in a suicide bombing that targeted a Shiite gathering in the Iraqi capital's al-Shaab district, according to police sources. ISIS claimed responsibility for the attack in a statement released by the ISIS-affiliated Amaq Agency.