Violence has sparked outside the city in recent days, and a strike on Sunday on one of Mosul's main bridges was seen as one of several indications the battle's launch was imminent.
Leaflets declaring "Victory Time" were reportedly dumped over the city on Sunday.
US troops are said to be playing a "supporting role" in the offensive, with the Iraqi army and Kurdish Peshmerga fighters making up the bulk of the 30,000-strong force.
— Brett McGurk (@brett_mcgurk) October 16, 2016
Washington recently announced the deployment of 600 additional US troops to help with the city's recapture, bringing the total number of US force management personnel to move than 5,000, according to the Pentagon.
— Brett McGurk (@brett_mcgurk) October 16, 2016
"There are no more major objectives after that," Navy Capt. Jeff Davis said of the Mosul offensive. "This is it. This is the last big holdout in Iraq" for Daesh.
Around half of the city's original population of more than 2 million remain in Mosul, and UN refugee officials say most of them could flee once fighting begins, which would create what one representative has called "one of the largest man-made displacement crises of recent times," according to CNN.
Camps are reportedly being set up to accommodate those who leave the city.
Mosul, Iraq's second biggest city, along with a number of other northern and western Iraqi cities and towns, was seized in 2014 during a Daesh offensive.
