Iraqi and Kurdish forces launched a long-awaited offensive Monday to drive the Islamic State group out of the country's second largest city, Mosul, but the operation could take weeks, if not months, and many fear a humanitarian crisis.

Columns of armored vehicles trundled down desert roads toward the city as U.S.-led airstrikes and heavy artillery echoed across the Ninevah plains.

Kurdish forces captured a number of small, largely unpopulated villages to the east. But their progress was slowed by roadside bombs left behind by the militants, and IS unleashed a series of suicide car and truck bomb attacks, one of which struck a Kurdish tank. It was not immediately clear if there were any casualties from that incident or fighting elsewhere.

FEARS OF A HUMANITARIAN CRISIS

Mosul, which fell to IS when the extremists swept across much of northern and central Iraq in the summer of 2014, is still home to a million people. Aid groups fear that the fighting could cause a mass exodus that would overwhelm nearby refugee camps.

The U.N. says the fighting could displace at least 200,000 people. Camps have been set up to receive the displaced, but aid groups say there is only room for around 100,000. Aid officials also fear that IS could use civilians as human shields.

"Civilians who attempt to escape the city will have little choice but to take their lives into their own hands and pray that they are able to avoid snipers, land mines, booby traps and other explosives," said Aleksandar Milutinovic, the International Rescue Committee's Iraq Director.

COALITION DIVISIONS

The Mosul offensive is the largest and most complex military operation launched in Iraq since U.S. forces withdrew in 2011. It involves an estimated 25,000 troops from the army, the Kurdish peshmerga, Sunni tribal forces and Shiite militias.

In the weeks leading up to the launch of the offensive, Iraq and Turkey became embroiled in a spat over the presence of some 500 Turkish troops near Mosul who have been training anti-IS fighters. Baghdad has demanded the Turkish troops withdraw, while Turkey has insisted it play a role in the liberation of the city.

For now the various forces are working together, but that could become more difficult as they close in on the city, and it remains unclear exactly how Mosul will be governed once IS is gone.