Iraqi government forces, with air and ground support from the U.S.-led coalition, launched an offensive on Monday to drive ISIS from the city of Mosul, the militants' last major stronghold in the country.

The assault on the northern city is the biggest operation mounted by the Iraqi military since U.S. forces withdrew in 2011, and the United States predicted the Islamic State of Iraq and Syria would suffer "a lasting defeat."

Long columns of armoured vehicles followed by hundreds of pickup trucks advanced on a cluster of some half-dozen villages on the plain outside the city. U.S.-led airstrikes and heavy artillery pounded the squat, dusty buildings. The area, historically home to religious minorities brutally oppressed by ISIS, was nearly empty of civilians, allowing air power to do much of the heavy lifting.

We are the real Muslims. — Unidentified Kurdish fighter

By mid-afternoon local time, a senior military commander said the assault was "going very well."

But Lt.-Col. Mohammad Darwish of the Kurdish forces known as Peshmerga said the main roads and fields were littered with homemade bombs and that suicide car bomb attacks had slowed the troops' progress.

The ISIS-run Aamaq news agency is claiming eight suicide attacks against the Peshmerga, and says ISIS destroyed two Humvees belonging to the Kurdish forces and Shia militias east of the city on Monday.

The Kurdish Rudaw TV, meanwhile, broadcast images of Kurdish tanks firing on two ISIS suicide truck bombs, one of which crashed and exploded.

Offensive aims to rid Iraqi city of ISIS 0:50

Some 30,000 troops from the Iraqi army, Peshmerga militia and Sunni tribal fighters were expected to take part in the offensive to drive out an estimated 4,000 to 8,000 ISIS militants.

The assault on the city of 1.5 million people could be one of the biggest military operations in Iraq since the 2003 U.S.-led invasion that toppled Saddam Hussein.

"This operation to regain control of Iraq's second-largest city will likely continue for weeks, possibly longer," said the commander of the coalition, U.S. Lt.-Gen. Stephen Townsend, in a statement.

Mosul is the largest city controlled by ISIS and its last major stronghold in Iraq.

'Decisive moment'

U.S. Defence Secretary Ash Carter said the entire U.S.-led international coalition stands ready to support Iraq "in the difficult fight ahead" to retake Mosul.

"This is a decisive moment in the campaign to deliver ISIS a lasting defeat," Carter said in a statement.

When we're done here, we will chase them to Raqqa. — Maj. Shiban Saleh

"We are confident our Iraqi partners will prevail against our common enemy and free Mosul and the rest of Iraq from ISIS's hatred and brutality."

In 2014, ISIS leader Abu Bakr al-Baghdadi proclaimed from Mosul's Grand Mosque a "caliphate" in Iraq and neighbouring Syria, meaning an Islamic state with himself its absolute ruler.

ISIS 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 Shia militias.

Reporter quickly ducks for cover as rapid gunfire rings out 0:25

The Iraqi Kurdish military command said 4,000 Peshmerga were taking part in an operation to clear several villages held by ISIS on the eastern front, in an attack co-ordinated with a push by Iraqi army units from the southern front.

"We are the real Muslims, Daesh are not Muslims, no religion does what they did," said a young Kurdish fighter, using another of the militant group's names, as he scanned the plain east of Mosul.

As he spoke, a Humvee drove by with the word "Rojava," or Syria's Kurdistan, painted on the protection plate of the machine gun turret.

"This is all Kurdistan,'' said Maj. Shiban Saleh, one of the fighters onboard. "When we're done here, we will chase them to Raqqa or wherever they go," he added referring to the largest city under control of the militants in Syria.

He said about 450 Syrian Peshermga fighters were involved in the offensive east of Mosul, which aims to take back nine villages during the day.

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Fears of sectarian violence

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.

Local Sunni politicians and regional Sunni-majority states including Turkey and Saudi Arabia cautioned that letting Shia militias take part in assault could spark sectarian violence.

"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," he added, asking the population to cooperate with the government's forces.

The Iraqi army dropped tens of thousands of leaflets over Mosul before dawn on Sunday, warning residents that the offensive was imminent.

The leaflets carried several messages, one of them assuring the population that advancing army units and airstrikes "will not target civilians." Others told residents to avoid known ISIS locations and "stay at home."

Aid workers fear as many as one million civilians could flee the city and its surrounding villages, according to freelance reporter Rebecca Collard, who spoke to CBC News Network from a refugee camp in Erbil, about 85 kilometres from the front lines.

Iraqi special forces soldiers prepare to move out from a camp near Khazer, Iraq, outside Mosul. (Adam Schreck/Associated Press)

More than three million people have been displaced since ISIS came to power two years ago.

"They have no place to put these people," Collard said. "Aid workers in the UN have said that they are just not going to be able to accommodate that sort of an influx."

Medicine is in short supply in Mosul, and food prices have risen sharply.

The UN humanitarian co-ordinator for Iraq says the operation to wrest Mosul from the group Islamic State in Iraq and Syria has the potential to become the single largest, most complex humanitarian operation in the world in 2016.

Speaking via video-link from Iraq, Lise Grande said Monday that in a worst-case scenario, some 700,000 civilians would require shelter, overwhelming emergency sites that currently only have the capacity to hold 60,000 people.

"Our capacity to support 700,000 people in the short-term — we couldn't do it. And certainly if we had to mount a response over the intermediate term, if they couldn't go back to Mosul quickly, if there was too much damage in the city, then it would test us to breaking point," Grande said.

Stephen O'Brien, under secretary general for humanitarian affairs, 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."