KHAZER, Iraq — Iraqi and Kurdish forces backed by U.S.-led airstrikes launched coordinated military operations early today as the long-awaited fight to wrest the northern city of Mosul from Islamic State fighters got under way. But the battle is likely to be long and it was unclear when the troops would enter the city itself.

The fate of more than a million civilians trapped inside Mosul also will be critical as the battle intensifies in the days and weeks ahead amid concerns that IS could use them as human shields.

Convoys of Iraqi, Kurdish and U.S. forces moved east of Mosul along the front line early today as U.S.-led coalition airstrikes sent plumes of smoke into the air and heavy artillery rounds rumbled in the distance.

Iraqi Prime Minister Haider al-Abadi announced the start of the operations on state television, launching the country’s toughest battle since American troops left nearly five years ago.

Mosul, Iraq’s second largest city, fell to IS in the summer of 2014.