Iraqi security forces have begun their attack on western Mosul, the day after recapturing the city's airport from the so-called Islamic State (IS) group.
The government is fighting to drive IS from the remains of its stronghold.
Heavy airstrikes took place overnight. Armoured columns of fighters have started their assault, which takes them from desert and farmland into a densely packed city.
IS remain in retreat after eastern Mosul was retaken last month.
"This is where the real fighting starts," a colonel leading the operation told the BBC of the assault on the city's west.
The BBC's Quentin Sommerville, who is embedded in the area with federal police, said this next stage of the battle would be even tougher, as the streets are narrow and the area is more heavily populated.
It also includes districts seen as pro-IS.
Leaflets warning residents of an imminent offensive were earlier dropped over the west of the city.
The UN has voiced concern about the welfare of civilians trapped in western Mosul.
Aram Shakaram, the country deputy director for Save the Children in Iraq, told the BBC he believed relatively few people had been able to escape since Wednesday.
He said the charity believed that nearly 800,000 people were still trapped there.
Taking the airport
Thursday's operation to take control of the airport took four hours, and left the army within less than a mile (1.6km) outside of western Mosul.
It began with air strikes by the US-led coalition before armoured columns advanced to the airport's perimeter.
The runway was found to have been destroyed by IS, but our correspondent says it still has value.
It is a large piece of land and controlling it will help secure southern routes to west Mosul, he says.
The airport and the al-Ghazlani base are on Mosul's southern outskirts on the western side of the Tigris river.
- Iraq gaining momentum against IS
- Islamic State group: The full story
'I could die in an air strike or be killed by IS' - Mohamed, activist
We are very scared, and we are worried that the final stages are going to take long.
IS will not give up and withdraw that easily and the whole operation might take as long as it did in east Mosul.
I won't lie, I am scared too and I don't know if I am going to die in an air strike or be killed by an IS fighter.
Voices from West Mosul: 'Everyone is scared'
Image copyright Reuters Image caption Iraqi forces have also attacked the nearby al-Ghazlani military base
More than 160,000 people have fled their homes in and around the city.
The UN said in late January that almost half of all the casualties in Mosul were civilians.
All bridges linking the east and west of the city, across the Tigris river, have been destroyed by air strikes.
IS jihadists overran Mosul as they spread across much of northern and western Iraq in 2014.