Senior Iraqi commander says the military operation is 'going very well'
Iraqi and Kurdish forces have seized a number of villages around Mosul - but their advance has been slowed by suicide bombers, roadside IEDs and oil fires.
The Peshmerga gained 80 square miles of territory from Islamic State during the first day of the huge military operation, according to the president of Iraq's Kurdish region.
Lt Col Mohammad Darkish of the Peshmerga said the main roads and fields were littered with homemade bombs.
Reporter Stuart Ramsay, who is embedded with the Kurdish fighters, said they were searching through outlying villages for IS fighters.
"They are literally going street-by-street trying to clear out any of the Islamic State if they are in here.
"They are not sure who is here - but they are being fired back at."
He also witnessed a coalition airstrike destroy a suspected suicide bomber in a truck who had been approaching the Kurdish forces.
An IS-run media outlet said the group had launched a series of suicide attacks on the advance, destroying at least two Humvees.
Retreating fighters also set oil alight throwing up thick black smoke to apparently cover their retreat and hide their positions from the aircraft flying overhead.
Some 2,000 Iraqi special forces are being supported by four brigades of the regular Iraqi Army, 15,000 Sunni militia, 15,000 Kurdish Peshmerga and a few thousand Shia militia.
They are taking the fight to an estimated 3,000 IS fighters in the country's second largest city.
Shelter for 700,000 civilians?
A senior Iraqi commander, Army Lt Gen Talib Shaghati, said some militants were fleeing with their families to Syria as the military pushed forward.
He said the military operation was "going very well".
A Pentagon official said Iraqi forces were "ahead of schedule" after the first day of the offensive, but warned it was unclear how long it would take to recapture the city.
"We are in the first day of what we assume will be a difficult campaign that could take some time," Peter Cook said.
Meanwhile, the UN humanitarian co-ordinator for Iraq has warned the battle may overwhelm emergency services as around 700,000 civilians could need shelter.
"Our capacity to support 700,000 people in the short-term - we couldn't do it," Lise Grande said.
"And certainly if we had to mount a response over the intermediate-term, if they couldn't go back to Mosul quickly, if there was too much damage in the city, then it would test us to breaking point."
Families have been advised by local forces to stay inside and put up a white flag on their homes to stay safe.
Mosul was overrun by IS in 2014 in what was described as a "total collapse" of government security forces, causing thousands of families to flee to Iraq's autonomous Kurdish region in the north.
IS leader Abu Bakr al Baghdadi visited the city to declare an Islamic caliphate, which at one point covered nearly a third of Iraq and Syria.
Using Mosul as a base, the jihadi group swept further south through Iraq's towns and cities.
But government soldiers trained by US forces have since gained ground and Mosul is now the last city in the country held by IS.