"Yesterday, our commander told us: This is it, the liberation of Mosul is beginning," a special forces sergeant told The Associated Press.
Iraqi officials have said their preparations for the Mosul offensive were coming to completion.
The Iraqi troops camped in a field across the Great Zab River separating Nineveh province from Iraq's Kurdistan region.
Nearby, other members of their unit were moving into an abandoned village, converting homes into a barracks for temporary camp.
The large volume of forces needed for the Mosul fight – Iraqi and coalition officials say at least 30,000 will take part – has overcrowded the few military bases along the Mosul front line.
The sergeant said his unit traveled in a convoy from Tikrit to the front near the district of Khazer, on the edges of the Nineveh plain about 30 kilometers (18 miles) from Mosul.
Qayyarah air base south of Mosul is expected to be the main hub of operations, but Iraqi army convoys, including a unit of the elite special forces, could also be seen moving to front-line positions east of the city.
The international coalition said Friday it also is conducting a larger number of airstrikes in the area, launching more than 50 in and around Mosul over the past two weeks.
Mosul, once home to more than 2 million people, fell to ISIS fighters more than two years ago. The operation to retake it is expected to be the most complex yet for Iraq's military.
In addition to Iraq's conventional military, other participants expected to participate in the battle are Shia militias, Kurdish forces known as the Peshmerga, Sunni tribal fighters and federal police forces.
Over the past week, coalition and Iraqi officials said they have seen small numbers of ISIS fighters fleeing Mosul into neighboring Syria.
The Syrian anti-IS activist group, Raqqa is Being Slaughtered Silently, on Thursday said that 10 buses and 35 SUVs with Iraqi license plates carrying ISIS fighters and their families arrived in the militants’ Syrian stronghold of Raqqa.
It's still unclear, however, if the militants will put up a strong fight for the city.
As Iraqi forces have moved to isolate Mosul, residents report that ISIS fighters have begun reinforcing trenches around the city and setting booby-trapped explosives along main thoroughfares.
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