The advance started with artillery and mortar strikes on the Aden, Tahrir, and Quds districts, just west of special forces' footholds in the Gogjali and Karama neighbourhoods, Lt. Col. Muhanad al-Timimi told The Associated Press. IS responded with mortar fire, he added, kicking off small arms clashes between the two sides. Smoke from artillery strikes rose over the city. The Islamic State group is fighting to hold Iraq's second city of Mosul as Iraqi forces and allied Kurdish troops squeeze in from all directions with US-led coalition support, mostly with airstrikes and reconnaissance. On Tuesday, Iraqi troops entered the city limits for the first time in more than two years — soldiers had withdrawn from Mosul in the face of the Islamic State group's 2014 blitz that seized large swaths of territory in Iraq and neighbouring Syria. Now the Iraqi forces are gearing up for urban warfare expected to take weeks, if not months, as they work their way neighborhood by neighbourhood, going through a warren of dense buildings prone to booby traps and ambushes. More than 1 million civilians are stuck in the city, complicating the military's efforts to advance without harming innocents. IS militants have driven thousands of them deeper into the city's built-up areas, presumably for use as human shields, while hundreds of others have fled in the past days toward government-controlled territory despite the uncertainty of resettlement in displacement camps. Mosul is the last major IS stronghold in Iraq, and expelling the militant group from the city would be a major blow to the survival of its self-declared 'caliphate' that stretches into Syria. When IS seized Mosul and other territory in 2014, the much larger Iraqi military had been neglected and demoralised by corruption. Iraqi forces have made uneven progress in closing in on the city. Advances have been slower to the south, with government troops still 35 kilometres (20 miles) away, although they seized a handful of villages late last week. Kurdish fighters and Iraqi army units are deployed to the north, while government-sanctioned Shiite militias are sweeping in from the west to try to cut off any IS escape route. One of the leading Shiite militias, the Hezbollah Brigades, said on Wednesday that its fighters have gained control of a highway linking Mosul to the Syrian city of Raqqa, the de facto capital of the militants' self-proclaimed caliphate. The militias' umbrella group, the Popular Mobilisation Units, say they will not enter Mosul and will instead focus on retaking Tal Afar, a town to the west that had a Shiite majority before it fell to IS in 2014.