By Isabel Coles
MOSUL, Iraq (Reuters) - U.S.-backed Iraqi forces pushed deeper into western Mosul on Saturday after retaking the city's airport from Islamic State and punching into built-up areas of the jihadists' last major stronghold in the country.
Troops advanced in several more populated, built-up southern districts and about 1,000 civilians walked across the frontlines, the largest displacement since fighting began a week ago in a bid to deal Islamic State a decisive blow.
The new offensive comes after government forces and their allies finished clearing Islamic State from eastern Mosul last month, confining the insurgents to the western sector of the city bisected by the Tigris river.
Commanders expect the battle in western Mosul to be more difficult, in part because tanks and armored vehicles cannot pass through the narrow alleyways that crisscross ancient districts of the city.
But Iraqi forces have so far made quick advances on multiple fronts, capturing the northern city's airport on Thursday, which they plan to use as a support zone, and breaching a three-meter high berm and trench set up by Islamic State.
The advancing forces are now less than three kilometers (two miles) from the mosque in Mosul's old city where Islamic State leader Abu Bakr al-Baghdadi declared a caliphate in 2014 spanning Iraq and Syria, sparking an international military campaign to defeat the group.
Losing Mosul would likely deal a hammer blow to the militants' dream of statehood, but they still control swathes of territory in Syria and patches of northern and western Iraq from where they could fight a guerrilla-style insurgency in Iraq, and plot attacks on the West.
Federal police and an elite Interior Ministry unit known as Rapid Response have completely recaptured Hawi al-Josaq just north of the airport and begun clearing houses in the Tayyaran district, said Brigadier General Hisham Abdul Kadhim.
Islamic State was resisting with snipers and roadside bombs, he said.
Separately, counter-terrorism forces were advancing on two directions towards Wadi Hajr and Mamoun districts, said Lieutenant General Abdelwahab al-Saadi, a senior commander.
"Clearing operations are ongoing and our forces have entered those areas," he told Reuters on a hill overlooking the battle.
Saadi said an Islamic State suicide car bomb had been destroyed on Saturday morning before reaching its target.
CIVILIANS START TO FLEE
The campaign involves a 100,000-strong force of Iraqi troops, Shi'ite militias and Sunni tribal fighters. It is backed by a U.S.-led coalition that provides vital air support as well as on-the-ground guidance and training.
Western advisors are increasingly present close to the frontline, helping coordinate air strikes and advising Iraqi forces as the battle unfolds.
About a thousand civilians, mostly women and children, walked out of southwestern parts of Mosul on Saturday and climbed into military trucks taking them to camps further south.
The United Nations says up to 400,000 people may have to leave their homes during the new offensive as food and fuel runs out in western Mosul. Aid groups warned on Friday that the most dangerous phase of the offensive was about to begin.
Some of the people fleeing the Mamoun area said they were originally from Hammam al-Alil, south of Mosul, but were forced to move four months ago as Islamic State retreated north into the city.
"They began shelling us arbitrarily, so we hid in the bathrooms. When the security forces came, they yelled to us so we fled to them," said civilian Mahmoud Nawwaf.
The government is encouraging residents to stay in their homes whenever possible, as they did in eastern Mosul where fewer people fled than expected.
A Reuters correspondent near the airport saw nine families living in a house where residents with full beards served trays of tea to security forces. Some said Islamic State had forced them to move from Samarra, 250 km (160 miles) south of Mosul.
Abu Naba, 37, said he was surprised at how quickly the militants had been driven out.
"We could hear their voices outside and 15 minutes later they were gone," he said.
A woman with a baby wrapped in a blanket on her lap said she had given birth in the house 22 days ago because it was too dangerous to reach a hospital.
(Additional reporting by Reuters TV south of Mosul; writing by Stephen Kalin; editing by David Clarke)