Advancing from several directions, the forces moved towards Mosul airport, which lies just south of the city, marking a new phase in Iraq's largest military operation in years.
The Islamic State group has put up stiff resistance to defend Mosul, the city where its leader Abu Bakr al-Baghdadi proclaimed a "caliphate" straddling Iraq and Syria in 2014.
"Our forces are beginning the liberation of the citizens from the terror of Daesh," Prime Minister Haider al-Abadi said in a short televised speech, using an Arabic acronym for IS.
"We launched our operation at 7:00 am (0400 GMT)... We are heading towards the airport," said Abbas al-Juburi of the interior ministry's elite Rapid Response force.
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The sky south of Mosul was black with smoke from air strikes and artillery as thousands of forces in armoured convoys worked converged on the airport.
"They're desperate," Ali, a Rapid Response officer, said in the village of Al-Buseif as helicopters flew overhead, tracking the last IS fighters attempting to flee.
The jihadists overran Mosul and swathes of other territory north and west of Baghdad in 2014, routing security forces ill -prepared to face the assault.
The Iraqi government launched the offensive to reconquer Mosul on October 17, throwing tens of thousands of forces into the long-awaited counter-attack with air and ground support from a US-led coalition fighting IS in Iraq and Syria.
The Joint Operations Command coordinating the fight against IS declared east Mosul "fully liberated" on January 24.
After a pause, federal forces now face what was always billed as the toughest nut to crack: Mosul's west bank, home to the narrow streets of the Old City.
"West Mosul had the potential certainly of being more difficult, with house-to-house fighting on a larger and more bloody scale," said Patrick Skinner, from the Soufan Group intelligence consultancy.
The coalition said it carried out a total of 40 air strikes on Saturday, nine of which struck targets in the Mosul area.