by RICHARD ENGEL and ALASTAIR JAMIESON – QAYYARAH, Iraq (NBC News)— Iraqi and Kurdish forces directed by U.S. advisers advanced on Mosul early Monday as a long-awaited offensive to end the city's brutal two-year occupation by ISIS got underway.
Onlookers cheered and waved as tanks rolled toward the city under sky blackened by burning oil wells deliberately set ablaze by retreating jihadis.
Morale was high and soldiers flashed victory signs as huge columns of military vehicles moved on the city. Some troops even danced.
"The hour has struck. The campaign to liberate Mosul has begun," Iraqi Prime Minister Haider al-Abadi said as he announced the move.
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.
While many militants have already fled, forces advancing from all sides of the city face danger from booby traps and IEDs. One Iraqi news crew witnessed a suicide attack targeting Kurdish Peshmerga forces.
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.
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.
A source inside Mosul told NBC News that ISIS militants and their families had "disappeared from most parts" in the east of the city and that unknown groups were trying to kill any remaining jihadis.
The hour has struck. The campaign to liberate Mosul has begun. Beloved people of Mosul, the Iraqi nation will celebrate victory as one— Haider Al-Abadi حيدر العبادي (@HaiderAlAbadi) October 17, 2016
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.
Stephen O'Brien, U.N. Under-Secretary General for Humanitarian Affairs, said he was "extremely concerned" that families were at risk of "being caught in cross-fire or targeted by snipers."
"Tens of thousands of Iraqi girls, boys, women and men may be under siege or held as human shields. Thousands may be forcibly expelled or trapped between the fighting lines," he said in a statement, calling on all parties in the conflict to "uphold their obligations under international humanitarian law."
Abadi said that ISIS would be "punished" for its crimes and that the province's cities and villages will be rebuilt.
"We will bring life back to Mosul and all other areas around Mosul," he added.
In a statement, Defense Secretary Ash Carter called the offensive a "decisive moment in the campaign to deliver [ISIS] a lasting defeat."
Brett McGurk, the State Department official coordinating the effort against group, said it would liberate Iraqis from "two years of darkness."
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 Brig. Gen Haider Fadhil told The Associated Press in an interview that more than 25,000 troops, including paramilitary forces made up of Sunni tribal fighters and Shiite militias, were taking part.
The role of the Shiite militias has been particularly sensitive, as Nineveh, where Mosul is located, is a majority Sunni province and Shiite militia forces have been accused of carrying out abuses against civilians in other operations in majority Sunni parts of Iraq.
Fadhil voiced concern about potential action from Turkish troops based in the region of Bashiqa, northeast of Mosul.
Turkey sent troops to the area late last year to train anti-ISIS fighters there. But Baghdad has seen the Turkish presence as a "blatant violation" of Iraqi sovereignty and has demanded the Turkish troops withdraw, a call Ankara has ignored.