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Iraqi forces launched a long-awaited offensive early Monday to push Islamic State from Mosul, as thousands of government troops and Kurdish fighters advanced toward the outskirts of the militant-held city.

Flanked by senior Iraqi military commanders, Prime Minister Haider al-Abadi went on national television to announce the start of the operation to retake Iraq's second-largest city from Islamic State, declaring, "The hour of victory has sounded and the Mosul liberation operation has started."

Within hours, the military said, Iraqi army troops were moving on the Bartalaa area at the edge of the northern Iraqi city, which has been under Islamic State control for more than two years.

Some 5,000 Kurdish Peshmerga forces in armored Humvees and other troop carriers were also advancing on the area, according to a local commander. U.S. troops working with the Kurds were aboard a number of the vehicles.

Peshmerga engineers in bulldozers built berms and dug trenches to protect newly-controlled Kurdish territory from Islamic State car bombers.

As the operation got under way, U.S. Defense Secretary Ash Carter called it a "decisive moment" in the campaign to defeat the militant group.

"The United States and the rest of the international coalition stand ready to support Iraqi Security Forces, Peshmerga fighters and the people of Iraq in the difficult fight ahead," Mr. Carter said.

Reclaiming Mosul is seen as essential to the broader battle against Islamic State. The Islamist militant group has suffered a string of losses recently in its self-declared caliphate, including on Sunday, when Syrian rebels backed by Turkey and the U.S. drove the militants from the Syrian town of Dabiq.

The assault on Mosul comes after months during which Iraqi forces have tried to capitalize on discontent with Islamic State's harsh rule to turn locals against the group and seek aid for Iraqi forces. In some cases, Iraq has supplied arms to residents.

Mass defections, internal rivalries and an increasing defiance among Mosul's estimated 1.2 million residents have contributed to a sense of confidence inside Iraq's military.

Still, Iraqi and American officials expect a tough fight, with a specific concern that a desperate group of fighters will attempt to use the city's inhabitants as human shields. Aid agencies and the United Nations are bracing for an expected exodus that could exceed their resources.

Stephen O'Brien, the U.N. under-secretary-general for humanitarian affairs and emergency relief coordinator, warned that the agency didn't have sufficient funds to prepare fully for "the worst-case scenario" of estimated one million displaced people.

"Families are at extreme risk of being caught in crossfire or targeted by snipers," he said.

The fight to reclaim Mosul is also a test of Iraq's ability to remain united. The city is majority Sunni Muslim, but is located in one of the country's most diverse regions and was once home to many of Iraq's smaller ethnic minorities.

Islamic State, a Sunni-led group, sent shock waves through the region when it captured Mosul in June 2014 in a blitz that saw Iraq's military and police melt away despite billions of dollars spent by the U.S. to train and equip government forces.

It was in Mosul later that month that Islamic State leader Abu Bakr al-Baghdadi made his first and only public appearance, giving a Friday prayer sermon in one of the city's grandest mosques. There he designated Mosul the self-declared caliphate's Iraqi capital, a home for both the group's leadership and families of thousands of its foreign fighters.

The Iraqi military has since recovered and is riding a wave of momentum in which they have evicted Islamic State from major cities including Sinjar, Beiji, Ramadi and Fallujah since the end of last year.

Kurdish Peshmerga fighters and local Sunni tribal fighters are participating in the Mosul offensive, while Iran-backed Shiite militias have been tasked with securing areas south of the city.

Just before Mr. Abadi's announcement, Peshmerga forces massed outside Mosul appeared buoyant and eager to launch the operation, as jet fighters and helicopters traversed the night sky.

Many fighters said they were unable to sleep, too excited over engaging Islamic State in what they considered their last stand against the militants.

Iraq's military said it expects Islamic State to put up a stiff resistance, using booby traps, suicide vehicle attacks, snipers and human shields to repel the onslaught, taking advantage of Mosul's dense urban architecture and population.

A mid-ranking Islamic State commander said in an interview over Facebook that the group has made a tactical decision to partially abandon Mosul, recalling their "human resources" to Syria where they hope to strengthen their foothold.

"There will be no big great epic battle in Mosul," the commander said. "The tactic now is hit-and-run."

Unlike previous fights to dislodge Islamic State from Iraqi cities, this fight is the first time Iraq's various armed groups have been tasked with working together.

In the weeks leading up to the battle, Iran-backed Shiite militias jockeyed for a more prominent role in the fight. They proved to be formidable soldiers in retaking other Iraqi cities, including Tikrit and Fallujah, earning political influence and popularity among Iraq's Shiite majority.

The militias' demands followed a public spat between Turkey and the Baghdad government over the presence of Turkish troops near Mosul. Ankara's insisted that its forces wouldn't withdraw, and would hold sway over the campaign to reclaim Mosul.

But Iraqi Sunnis and the U.S. have tried to keep the militias away from Iraq's Sunni heartland following allegations of widespread revenge killings and detentions of Sunni men the militias accuse of being Islamic State sympathizers.

Eleventh-hour negotiations in early October by Mr. Abadi with the disparate armed groups restored the original battle plan, which calls for only Iraq's military and allied Sunni tribal fighters to enter the city.

Write to Tamer El-Ghobashy at tamer.el-ghobashy@wsj.com and Ben Kesling at benjamin.kesling@wsj.com

(END) Dow Jones Newswires

October 17, 2016 07:35 ET (11:35 GMT)

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