KHAZER, Iraq — Columns of Iraqi and Kurdish forces backed by U.S.-led airstrikes slowly advanced on Mosul from several directions on Monday, launching a long-awaited operation to retake Iraq’s second largest city from the Islamic State group.

As airstrikes sent plumes of smoke into the air and heavy artillery rounds rumbled, troops pushed into abandoned farming villages on the flat plains outside the city. But they were slowed by roadside bombs and by suicide car and truck bombs hurled at them by the militants.

The unprecedented operation is expected to take weeks, even months. Though some of the forces are less than 30 kilometers (20 miles) from Mosul’s edges, it was not clear how long it will take to reach the city itself. Once there, they have to fight their way into an urban environment where more than 1 million people still live.

Aid groups have warned of a mass exodus of civilians that could overwhelm refugee camps.

Iraqi Prime Minister Haider al-Abadi announced the start of the operations on state television, launching the country’s toughest battle since American troops withdrew from Iraq nearly five years ago.

“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,” al-Abadi said, addressing the city’s residents and using the Arabic acronym for IS. “God willing, we shall win.”

Mosul, Iraq’s second largest city, fell to IS in the summer of 2014 as the militants swept over much of the country’s north and central areas. Weeks later the head of the extremist group, Abu Bakr al-Baghdadi, announced the formation of a self-styled caliphate in Iraq and Syria from the pulpit of a Mosul mosque.

If successful, the liberation of Mosul would be the biggest blow yet to the Islamic State group. Al-Abadi pledged the fight for the city would lead to the liberation of all Iraqi territory from the militants this year.

In Washington, Defense Secretary Ash Carter called the launch of the Mosul operation “a decisive moment in the campaign” to defeat IS. The U.S. is providing airstrikes, training and logistical support, but insists Iraqis are leading the charge.

More than 25,000 troops will be involved in the operation, launching assaults from five directions, according to Iraqi Brig. Gen. Haider Fadhil. The troops include elite special forces who are expected to lead the charge into the city, as well as Kurdish forces, Sunni tribal fighters, federal police and state-sanctioned Shiite militias.

The Kurdish forces, known as peshmerga, advanced in long columns of armored vehicles followed by hundreds of pickup trucks on a cluster of some half dozen villages east of the city on Monday.

Airstrikes and heavy artillery pounded the squat, dusty buildings. The area — historically home to religious minorities brutally oppressed by IS — was almost completely empty of civilians, allowing air power to do much of the heavy lifting.

But Lt. Col. Mohammad Darwish said the main roads and fields were littered with homemade bombs and that suicide car bomb attacks slowed progress.

Fighters entered the villages in Humvees but did not get out of their vehicles because it was too dangerous, a Peshmerga major said, speaking on condition of anonymity as he was not authorized to brief the press.