Iraqi forces have launched a long-awaited offensive to retake west Mosul, making cautious advances towards the city’s airport ahead of a push into heavily populated residential areas, where up to 3,000 Islamic State fighters have vowed to defend their last urban stronghold in Iraq.

An estimated 650,000 civilians are trapped in the west of the city, and the UN said on Sunday that many people were “at extreme risk”, with fuel and food supplies dwindling and drinking water and electricity scarce.

Plumes of smoke were seen as US airstrikes supported the early advance, which met limited resistance as forces probed areas of south and south-west Mosul. More dogged clashes are expected when troops near the urban centre, which is known to have been fortified by members of Isis’s elite fighting units.

Senior officers said the battle was likely to take at least as long as the grinding fight for the east of the city, which began in mid-October and ended in January when battered forces reached the Euphrates river, which bisects east and west.

Airstrikes and acts of sabotage have destroyed four of the five bridges across the river, with the last remaining bridge able to be crossed only on foot. Isis gunmen have shot at anyone who has tried to make the journey in recent weeks, making the crossing all but impossible.

Iraq’s prime minister, Haidar al-Abadi, announced the launch of the west Mosul offensive early on Sunday, minutes before up to 30,000 Iraqi police and soldiers began the operation. He asked the Iraqi armed forces to respect human rights during the battle and to take care of those displaced by the fighting.

Iraqi officials believe more residents of western Mosul are ideologically aligned with the terror group than in the east. The extremists are known to have prepared an extensive network of ambush points in the area’s narrow streets and alleys. But senior Isis members have fled the city in recent weeks, with some melting back into Anbar province and others making for the Syrian border, where they have joined other senior officials in villages to the south of Raqqa.