Two years after the jihadists seized the city of 1.5 million people and declared a caliphate from there encompassing tracts of Iraq and Syria, a force of some 30,000 Iraqi and Kurdish Peshmerga forces and Sunni tribal fighters began to advance.
Helicopters released flares and explosions could be heard on the city’s eastern front, where Reuters watched Kurdish fighters move forward to take outlying villages.
A US-led air campaign has helped drive the IS group from much of the territory it held but 4,000 to 8,000 fighters are thought to remain in Mosul.
Residents contacted by phone dismissed reports on Arabic television channels of an exodus by the jihadists, who have a history of using human shields and have threatened to unleash chemical weapons.
“Daesh are using motorcycles for their patrols to evade air detection, with pillion passengers using binoculars to check out buildings and streets,” said Abu Maher, using an Arabic acronym for the IS group.
He and others contacted were preparing makeshift defences and had been stockpiling food in anticipation of the assault, which officials say could take weeks or even months. The residents withheld their full names for security reasons and Reuters was not able to verify their accounts independently.
But the offensive, which has assumed considerable importance for US President Barack Obama as his term draws to a close, is fraught with risks.
These include sectarian conflict between Mosul’s mainly Sunni population and advancing Shiite forces, and the potential for up to a million people to flee Mosul, multiplying a refugee crisis in the region and across Europe.
The commander of the coalition, US Lieutenant General Stephen Townsend, said the operation to take Iraq’s second largest city would likely continue for weeks, “possibly longer”.
If Mosul falls, Raqqa in Syria will be the IS group’s last city stronghold.
“This is a decisive moment in the campaign to deliver ISIL a lasting defeat,” US Defense Secretary Ash Carter said in a statement, using another acronym for the Islamic State group.
“We are confident our Iraqi partners will prevail against our common enemy and free Mosul and the rest of Iraq from ISIL’s hatred and brutality.”
IS group has been retreating since the end of last year in Iraq, where it is battling US-backed government and Kurdish forces as well as Iranian-backed Iraqi Shiite militias.
But in a blow to the anti-IS group campaign, last week the jihadists crushed a planned rebellion within its ranks in Mosul intended to ease the recapture of the city.
The Iraqi Kurdish military command said 4,000 Peshmerga were taking part in an operation to clear several villages held by the IS group to the east of Mosul, in an attack coordinated with a push by Iraqi army units from the southern front.
The Iraqi army media office said advancing troops destroyed a number of IS group defence lines and strikes by Iraqi and coalition jets hit militant positions.
A column of black smoke was rising from one IS group position on the eastern front, a Reuters correspondent said, apparently from burning oil being used to block the path of the Kurds and obstruct the jets’ view.
“We are the real Muslims, Daesh are not Muslims, no religion does what they did,” said a young Kurdish fighter in battle dress as he scanned the plain east of Mosul from his position on the heights of Mount Zertik.
As he spoke a Humvee drove by with the word Rojava, or Syria’s Kurdistan, painted on the protection plate of the machine gun turret.
“This is all Kurdistan,” Major Shiban Saleh, one of the fighters onboard, said. “When we’re done here, we will chase them to Raqqa or wherever they go,” he said.
The United Nations last week said it was bracing for the world’s biggest and most complex humanitarian effort in the battle for Mosul, which could leave up to 1 million people homeless and see civilians used as human shields or even gassed.