The operation Iraqi forces launched on Monday to retake the city of Mosul from the Islamic State group could last weeks and "possibly longer", a top US general said.
Lieutenant General Stephen Townsend, commander of the US-led coalition battling the jihadist group, warned that victory in IS's last major stronghold in Iraq could take time.
"This operation to regain control of Iraq's second-largest city will likely continue for weeks, possibly longer," he said in a statement.
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Iraqi Prime Minister Haider al-Abadi announced earlier Monday that the long-awaited operation to recapture Mosul was under way.
"This may prove to be a long and tough battle, but the Iraqis have prepared for it and we will stand by them," Townsend said.
Following the fall of Mosul to IS more than two years ago, the United States led what has now become a more than 60-nation coalition supporting the anti-IS effort in Iraq and Syria.
Most of the coalition's support has come in the shape of air strikes and training but US, French and British special forces are now also on the ground to advise local forces in battle.
Townsend said the coalition had trained and equipped more than 54,000 Iraqi forces.
Meanwhile thousands of Iraq's Kurdish peshmerga forces advanced on jihadist-held villages east of Mosul on Monday as part of a broad operation to retake the city from the Islamic State group.
"The operation in Khazir includes up to 4,000 peshmerga in three fronts to clear nearby IS-occupied villages," the general command of the peshmerga said in a statement.
It said the operation was coordinated with Iraqi federal forces moving from the south and had received extensive air support from the US-led coalition.
Peshmerga commanders said the push was the third phase of an effort that started months ago to retake villages in the Nineveh plain that were captured by IS in 2014 and used to be inhabited by members of the Christian and Kakai minorities.
The latest offensive by the peshmerga is part of a broader operation to retake Mosul announced by Iraqi Prime Minister Haider al-Abadi early on Monday.
The much-anticipated announcement comes more than two years after IS seized Iraq's second city, paving the way for the proclamation of its "state".
Mosul is the extremist group's last major stronghold in Iraq and its recapture would shatter IS's claim that it is still running a "caliphate" straddling Iraq and Syria.
Abadi stressed in his televised address that only army and police forces would enter Mosul itself when advancing forces eventually reach the city's boundaries.
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