Iraqi Forces Prepare to Retake Mosul From ISIS

Smoke rises from clashes in the east of Mosul during clashes with Islamic State militants in Iraq on Oct. 17, 2016.

Iraqi and Kurdish forces backed by coalition airstrikes advanced on Mosul early Monday as a long-awaited offensive to end the city's brutal two-year occupation by ISIS got underway.

Peshmerga forces advance in the east of Mosul to attack Islamic State militants in Mosul on Oct. 17.

"The hour has struck. The campaign to liberate Mosul has begun," Iraqi Prime Minister Haider al-Abadi said as he announced the move.

Iraqi policemen clean a weapon at the Qayyarah military base, about 35 miles south of Mosul on Oct. 16.

Brett McGurk, the State Department official coordinating the effort against the group, said it would liberate Iraqis from "two years of darkness."

Iraqi forces deploy in the area of al-Shourah, about 28 miles south of Mosul, as they advance towards the city to retake it from the Islamic State group jihadists on Oct. 17.

The massive and complex military operation will be the largest in Iraq since American troops left in 2011 and, if successful, the biggest blow yet to ISIS.

An Iraqi policeman tries on a gasmask at the Qayyarah military base on Oct. 16.

Iraqi Kurdish Peshmerga fighters hold a position on the top of Mount Zardak, about 15 miles east of Mosul on Oct. 17.

Peshmerga forces advance in the east of Mosul on Oct. 17.

Brigadier Helgord Hekmat, a spokesman for the Kurdish forces, said 4,000 Peshmerga launched from about 60 miles to the east of the city and had retaken a cluster of villages."They are advancing to Bartella and liberating all areas on the way to that location," he told NBC News.

A Peshmerga convoy drives towards a frontline in Khazer on Oct. 17.

Thousands of Iraq's Kurdish peshmerga forces advanced on jihadist-held villages east of Mosul. On the other side of the city, Iraqi forces were advancing from the west and had reached Hamdaniyah, about 100 miles from the city, according to a Joint Operation Command spokesman.

Iraqi Kurdish Peshmerga fighters hold a position on the top of Mount Zardak on Oct. 17.

Lt. Gen. Stephen Townsend, commander of Combined Joint Task Force Operation Inherent Resolve, said in a statement that the operation could take "weeks, possibly longer."

Iraqi security forces sleeps in a military vehicle on Oct. 15.

Peshmerga forces gather on the east of Mosul on Oct. 17.

Iraq's second-largest city has been under ISIS rule for more than two years since government forces retreated. It is still home to up to 1.5 million civilians, according to U.N. estimates.