Iraqi soldiers, Kurdish fighters and Shia militiamen begin an offensive to retake Mosul from so-called Islamic State (IS)
The city is the jihadist group's last major urban stronghold in Iraq
A US-led coalition commander supporting the attack says the assault could "continue for weeks, possibly longer"
Reports from Mosul suggest IS fighters have prepared for the offensive with tunnels, suicide bombers, and possibly chemical weapons
IS claims a number of suicide attacks launched against the attacking forces
Alastair Lawson, David Molloy and Mal Siret
The BBC's Orla Guerin, who is with Kurdish Peshmerga forces, reported hours after the operation started that black smoke was rising from villages on the frontline east of Mosul.
"We assume that's because IS fighters inside have lit fires - possibly oil fires - to try and obscure visibility and to try and impede the coalition jets," she said.
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Iraqi pro-government forces have launched an operation to drive so-called Islamic State from Mosul, the country's second city.
About 30,000 soldiers, Kurdish Peshmerga fighters, Shia paramilitary forces and Sunni Arab tribesmen are involved in the offensive.
IS seized Mosul during a lightning offensive in June 2014.
The UN has said it is extremely concerned for the safety of the estimated 1.5 million people still living in the city, who have been forbidden by the jihadist group from leaving.