IRBIL, Iraq (AP) — Iraqi Prime Minister Haider al-Abadi announced the start of military operations to liberate the northern city of Mosul from Islamic State militants on Monday, launching the country on its toughest battle since American troops left nearly five years ago.

State TV aired a brief statement in the early hours Monday announcing the start of the widely anticipated military offensive to drive IS out of Iraq’s second-largest city.

Broadcasts showed the prime minister, dressed in the uniform of the elite counterterrorism forces, speaking while flanked by senior military officers.

“These forces that are liberating you today, they have one goal in Mosul which is to get rid of Daesh and to secure your dignity. They are there for your sake,” he told the city’s residents, using an alternate name for the militant group. “God willing, we shall win.”

The thuds of sporadic artillery shelling rumbled across the rolling Nineveh plains in the direction of Mosul, witnesses said. State TV broadcast patriotic music within minutes of the announcement.

The push to retake Mosul will be the biggest military operation in Iraq since American troops left in 2011 and, if successful, the strongest blow yet to the Islamic State. A statement on Al-Abadi’s website pledged the fight for the city marked a new phase that would lead to the liberation of all Iraqi territory from the militants this year.

Iraqi forces have been massing around the city in recent days. They include members of the elite special forces, who are expected to lead the charge into the city itself.

Mosul is home to more than a million civilians. The city fell to IS fighters during a lightning charge in June 2014 that left nearly a third of Iraq in militants’ hands and plunged the country into its most severe crisis since the U.S.-led invasion in 2003.

After seizing Mosul, IS leader Abu Bakr al-Baghdadi visited the city to declare an Islamic caliphate that at one point covered nearly a third of Iraq and Syria.

But since late last year, the militants have suffered battlefield losses in Iraq and their power in the country has largely shrunk to Mosul and small towns in the country’s north and west. Mosul is about 360 kilometers (225 miles) northwest of the capital, Baghdad.

The operation to retake Mosul is expected to be the most complex yet for Iraq’s military, which has been rebuilding from its humiliating 2014 defeat.

Iraqi forces began moving into Nineveh province to surround Mosul in July, when ground troops led by the country’s elite special forces retook Qayara air base south of the city. Thousands of Iraqi troops were deployed there ahead of the planned operation along with large numbers of tanks, armored personnel carriers and heavy artillery.

The base is ringed by a series of trenches, sand berms and other fortifications.

Iraqi troops were also positioned east of Mosul in the Khazer area, along with Kurdish Peshmerga forces, and to the north of the city near the Mosul Dam and Bashiqa areas.

Before the prime minister’s announcement, Brig. Gen Haider Fadhil told The Associated Press in an interview that more than 25,000 troops, including paramilitary forces made up of Sunni tribal fighters and Shiite militias, will take part in the offensive that will be launched from five directions around the city.

In addition to carrying out airstrikes, the U.S.-led international coalition will also offer artillery fire, he added. American troops are providing logistical support to the operation.

Brett McGurk, the U.S. envoy to the coalition against IS, said Abadi’s order called for “major operations” to free the city.

“Godspeed to the heroic Iraqi forces, Kurdish #Peshmerga, and #Ninewa volunteers. We are proud to stand with you in this historic operation,” he tweeted.

The role of the Shiite militias has been particularly sensitive, as Nineveh is a majority Sunni province and Shiite militia forces have been accused of carrying out abuses against civilians in other operations in majority Sunni parts of Iraq.

According to U.N. estimates, up to 1 million people could be displaced from Mosul during the operation, exacerbating the humanitarian situation in the country. The conflict in Iraq has forced 3.3 million people to flee their homes, with most of them living in camps or informal settlements.

Fadhil voiced concern about potential action from Turkish troops based in the region of Bashiqa, northeast of Mosul. Turkey sent troops to the area late last year to train anti-IS fighters there.

But Baghdad has seen the Turkish presence as a “blatant violation” of Iraqi sovereignty and has demanded the Turkish troops withdraw, a call Ankara has ignored.

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Associated Press journalists Ahmed Sami in Baghdad, Qassim Abdul-Zahra in Khazer, Iraq, and Vivian Salama in Washington contributed reporting.