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.
The IS-run new agency, Aamaq, said the group carried out eight suicide attacks against Kurdish forces and destroyed two Humvees belonging to the Kurdish forces and Shiite militias east of the city.
The Kurdish Rudaw TV broadcast images of Kurdish tanks firing on two trucks it said were IS suicide attackers. One of the trucks crashed into a tank and exploded. There was no immediate word on casualties from that attack or other fighting on Monday.
Just outside Baghdad - more than 225 miles (360 kilometers) southeast of Mosul - a suicide car bomber hit a checkpoint of security forces in the town of Youssifiyah, killing at least 12 people and wounded more than 30, officials said. They spoke on condition of anonymity as they were not authorized to talk to the media.
Iraqi army Lt. Gen. Talib Shaghati told The Associated Press that the Mosul operation "is going very well," but declined to give details. He said intelligence reports indicated that IS militants were fleeing toward Syria with their families.
IS once controlled nearly a third of Iraq and neighboring Syria. But over the past months, their territory has been dramatically reduced. In Iraq, their control is now limited to the area around Mosul and a few other small pockets.
For the Iraqi military, the battle is a test after two years of trying to rebuild from the humiliating defeat it suffered in the face of the IS blitz in 2014.
Mosul is also a test for the government's ability to control the multiple sectarian and ethnic tensions swirling around the conflict.
Mosul is a mostly Sunni city that was long a center of bitterness against the Shiite-led government, fueling insurgent and militant movements ever since the U.S.-led invasion of Iraq in 2003. While two years of IS rule may have left residents hating the militants, there is also likely little love for the government.
The role of the Shiite militias in the offensive has been particularly sensitive. Shiite militia forces have been accused of carrying out abuses against civilians in other Sunni areas.
But Sunnis are also suspicious of the Kurds, who have ambitions to expand their self-rule area into Ninevah province, where Mosul is located.
Lt. Col. Amozhgar Taher, with the peshmerga, said his men would not enter Mosul itself because of "sectarian sensitivities." Instead they will retake the villages to east of the city, home to Christians and the Shabak, another minority group.
Iraqi special forces Lt. Col. Ali Hussein said the Kurdish forces are leading the first push on Mosul's eastern front. His men will likely wait another day or two near the town of Khazer.
U.S. Lt. Gen. Stephen Townsend, commander of the anti-IS coalition, said in a statement it could take "weeks, possibly longer" to gain control of Mosul.
Military operations are predicted to displace 200,000 to a million people, according to the United Nations. Near the eastern front line, rows of empty camps line the road, ready to take in people fleeing. But aid groups say they only have enough space for some 100,000 people.
In Geneva, a senior U.N. humanitarian official, Undersecretary-General Stephen O'Brian, said he's "extremely concerned" for the safety of civilians in Mosul.
He said families are at "extreme risk" of being caught in crossfire, tens of thousands may end up besieged or held as human shields, and thousands could be forcibly expelled.
In the midst of financial crisis, the Iraqi government says it lacks the funds to adequately prepare for the humanitarian fallout. In some cases commanders say they are encouraging civilians to stay in their homes rather than flee.
"While we may be celebrating a military victory" after Mosul is liberated, "we don't want to have also created a humanitarian catastrophe," said Falah Mustafa, the foreign minister for Iraq's Kurdish region.
Schreck reported from Irbil. Associated Press journalists Ahmed Sami and Sinan Salaheddin in Baghdad, Qassim Abdul-Zahra in Khazer, Iraq, Maamoun Youssef in Cairo, Geir Moulson in Berlin, Suzan Fraser in Ankara, Turkey, and Vivian Salama in Washington contributed to this report.