Iraq's Prime Minister Haider al-Abadi on Monday launched a battle to liberate the northern city of Mosul from Islamic State in its most ambitious campaign since U.S. forces left five years ago, and the United States predicted the militant group would suffer "a lasting defeat."
"I announce today the start of the heroic operations to free you from the terror and the oppression of Daesh," he said in a speech on state TV, using an Arabic acronym for Islamic State.
"We will meet soon on the ground of Mosul to celebrate liberation and your salvation," he said, surrounded by the armed forces' top commanders.
Qatar-based al-Jazeera television aired video of what it said was a bombardment of Mosul that started after Abadi's speech, showing rockets and bursts of tracer bullets across the night sky and loud sounds of gunfire.
The assault on Mosul, with a population of 1.5 million, is backed by the U.S.-led coalition, which is providing air and ground support.
The offensive could be one of the biggest military operations in Iraq since the 2003 U.S.-led invasion that toppled Saddam Hussein and the biggest mounted by the Iraqi government since U.S. military forces withdrew in 2011.
About 30,000 troops were expected to take part from the Iraqi army, Kurdish Peshmerga militia and Sunni tribal fighters, while estimates of Islamic State forces in the city range from 4,000 to 8,000.
"This is a decisive moment in the campaign to deliver ISIL a lasting defeat," U.S. Defense Secretary Ash Carter said in a statement, using an acronym for Islamic State.
"We are confident our Iraqi partners will prevail against our common enemy and free Mosul and the rest of Iraq from ISIL's hatred and brutality."
Mosul is the largest city that Islamic State has controlled and its last major stronghold in Iraq. In 2014, Islamic State leader Abu Bakr al-Baghdadi proclaimed from Mosul's Grand Mosque a "caliphate" in Iraq and neighboring Syria, meaning an Islamic state with himself its absolute ruler.
The group has been retreating since the end of last year in Iraq, where it is confronting U.S-backed government and Kurdish forces as well as Iranian-backed Iraqi Shi'ite militias.
Speaking in the early hours of Monday, Abadi sought to allay fears that the operation would turn into sectarian bloodletting, saying that only the Iraqi army and police would be allowed to enter the mainly Sunni city.
"The forces that lead the liberation operation are the brave Iraqi army with the police forces," Abadi said. "They will enter the city and no one else."
The Iraqi army dropped tens of thousands of leaflets over Mosul before dawn on Sunday, warning residents that the offensive to recapture the city was imminent.
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.
Reflecting authorities' concerns over a mass exodus that would complicate the offensive and worsen the humanitarian situation in the area, the leaflets told residents "to stay at home and not to believe rumors spread by Daesh" that could cause panic.
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.
The United Nations 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.
There are already more than 3 million people displaced in Iraq in connection with conflicts with Islamic State. Medicine is in short supply in the city, and food prices have risen sharply.
U.N. Under Secretary General for Humanitarian Affairs Stephen O'Brien in a statement called for protection of Mosul's civilians, urging all parties to "ensure they have access to the assistance they are entitled to and deserve."