(CNN) A long-awaited operation to seize the key Iraqi city of Mosul after two years of ISIS control is imminent, Prime Minister Haider al-Abadi announced Sunday.

"We have been battling ISIS for more than two years," Abadi said during a televised meeting with journalists and political analysts. "We started fighting ISIS in the outskirts of Baghdad, and thank God we are now fighting them in the outskirts of Mosul, and God willing the decisive battle will be soon."

The announcement came hours after Iraqi forces dropped leaflets over Mosul, telling residents not to panic and to seal their doors and windows.

"We promise you that victory is near and that it will be a great victory fitting with the greatness of Iraq and its history and its people," Ahmed al-Assadi, a lawmaker and spokesman for the Popular Mobilization Units, or PMUs, which include dozens of militias fighting together, said earlier.

Witnesses on Sunday said airstrikes had taken out one of the city's main bridges, the latest move as forces warm up for the much-anticipated offensive on Mosul, a key city in the ISIS network and the militant group's last bastion of power in Iraq.

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It was unclear who was behind the airstrikes in Iraq's second-largest city, though Amaq, the news agency affiliated with ISIS, blamed US warplanes in a statement online.

The operation to liberate Mosul is expected to be a "messy and prolonged battle," CNN senior international correspondent Ben Wedeman said in the Iraqi city of Irbil, adding that the battle would likely come down to "street-to-street fighting."

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The PMU groups are fighting alongside the Iraqi military and are made up of mostly Shiites but also Sunnis, Christians and other ethnic and religious groups.

The majority of the PMU groups are mobilizing in coordination with Iraq's Internal Security Forces and the joint command for the looming battle, Assadi said. Forces have been moving toward Mosul and preparing for this battle -- to take place from the air and on the ground -- for more than 15 months.

'Victory time'

Sunday's airstrikes badly damaged al-Hurriya Bridge, though they appeared to be targeting four boats in the Tigris River. The bridge was not directly hit, but one of its columns was damaged, witnesses said.

The bridge is one of five connecting the two parts of the city separated by the Tigris.

ISIS militants blocked both ends of the bridge to prevent people from using after it was hit.

The militia's announcement that the battle would begin in days echoes a stern warning delivered by the Iraqi air force, which dropped thousands of leaflets on Mosul overnight.

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The four-page leaflets told residents that Iraqi-led forces, supported by the US-led coalition against ISIS, were making advances on the city and that no one should panic.

Residents were 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 was included.

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

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

ISIS executes some who flee

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

ISIS also 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.

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.

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

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

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.

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

Waiting for zero hour

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

PMU members 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.

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

Kurdish Peshmerga forces also are readying for the battle. 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

"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," Barzani said in a statement

Suicide bombing in Baghdad

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

On 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 Amaq.