Kurdish commanders claimed wide advances as Peshmerga fighters and Iraqi forces, backed by U.S.-led coalition jets, opened their assault on Mosul, part of a long-awaited effort to retake the northern Iraqi city from Islamic State militants.
After a day of artillery fire, bombs, and missiles that sent plumes of smoke into the air, Kurdish forces said on October 17 that they had retaken around 200 square kilometers and pushed back Islamic State fighters.
The battle to recapture Mosul, which has been under Islamic State control since 2014, is the largest military operation in Iraq since U.S. troops left in 2011.
Both Iraqi and U.S. officials are hoping a victory will deal a decisive blow to the extremist fighters, who stunned Iraq and Syria by seizing vast swaths of territory two years ago.
Government forces moved on the city from the Qayyarah base, some 60 kilometers south of Mosul, while Kurdish Peshmerga fighters advanced from the east.
The gains on the first day of battle appeared largely symbolic, as the allied forces moved into villages that were mainly empty of inhabitants. But U.S. Defense Department spokesman Peter Cook said it appeared the Iraqi forces were ahead of schedule so far.
"Early indications are that Iraqi forces have met their objectives so far and that they are ahead of schedule for this first day. This is going according to the Iraqi plan, but again, it's early, and the enemy gets a vote here," Cook told reporters in Washington.
WATCH: Iraqi, Kurdish Troops Begin Operation To Liberate Mosul
"We will see whether [Islamic State] stands and fights. We are confident no matter what, however, that the Iraqis have the capabilities to get this job done and we stand ready to support them along with the rest of the coalition," he said.
Earlier, Prime Minister Haidar al-Abadi, dressed in a military uniform and surrounded by Iraqi officers, announced the beginning of the operation with great fanfare.
"The hour of victory has come," Abadi said.
He also vowed that only government forces would enter Mosul, a Sunni-majority city. That's important because Iranian-backed Shi'ite militias have been taking part in operations around Mosul. Those militias have been accused of abuses against Sunni civilians, raising concerns about the treatment of Sunni civilians in Mosul.
"The force leading liberation operations is the brave Iraqi Army with the national police, and they are the ones that will enter Mosul, not others," Abadi said.
The effort to retake Mosul stands as a key military goal for the Abadi government but also a crucial political one, to show that the central government is capable of uniting some of the country's disparate ethnic factions and rival clans behind a common goal.
Iraqi Brigadier General Haidar Fadhil told the Associated Press that more than 25,000 troops were taking part in the offensive.
Iraqi forces deploy in the area of Al-Shurah, some 45 kilometers south of Mosul, as they advance toward the city on October 17.
The general command of the Peshmerga said the operation also included up to 4,000 Kurdish fighters who were seizing control of IS-held villages east of Mosul.
It said the operation was coordinated with Iraqi federal forces moving from the south and had received extensive air support from the U.S.-led coalition.
Lieutenant General Stephen Townsend, commander of the U.S.-led coalition forces, said the campaign would likely continue for weeks, possibly longer.
U.S. Secretary of Defense Ash Carter said the operation was key to defeating the IS militants.
A car-bomb attack is seen to the east of Mosul during clashes with Islamic State militants on October 17.
The United Nations has expressed "extreme concern" for the safety of up to 1.5 million people in and around Mosul.
"This is a decisive moment in the campaign to deliver” a lasting defeat to IS, Carter said.
"We are confident our Iraqi partners will prevail against our common enemy and free Mosul and the rest of Iraq" from the "hatred and brutality" of the extremists, he said.
UN Deputy Secretary-General for Humanitarian Affairs and Emergency Relief Stephen O'Brien said as many as 1 million people could be forced to flee their homes because of the operation.
"Tens of thousands of Iraqi girls, boys, women, and men may be under siege or held as human shields," O'Brien said in a statement. "Thousands may be forcibly expelled or trapped between the fighting lines."
Thousands of leaflets were dropped in recent days warning residents that the operation would be under way soon.
With reporting by AFP and AP