Iraqi forces are expected to target Mosul airport which lies on the west bank of the Tigris River in an attack touted as the toughest stage in efforts to oust IS from the city.
"Our forces are beginning the liberation of the citizens from the terror of Daesh," Iraqi Prime Minister Haider al-Abadi said in a short televised speech, referring to IS by its Arabic acronym.
"We announce the start of a new phase in the operation. We are coming, Nineveh [Mosul province], to liberate the western side of Mosul," he said.
The operation began with hundreds of leaflets being dropped on the IS-held section of the city calling for Iraqi fighters to surrender.
They also warned Iraqis in Mosul that this was there "last chance" to stop working with IS.
"To those of you who were intrigued by the IS ideology this is your last opportunity to quit your work with IS and to leave those foreigners who are in your homeland. Stay at home, raising the white flags as the forces approach," one leaflet read, according to the New York Times.
West Mosul in particular has been a stronghold of IS and its jihadi predecessors, and although the section of the city is smaller than the east bank it is much more densely packed.
The operation to capture West Mosul marks a significant step in Baghdad's plans to capture the remaining territories in northern Iraq held by IS.
Mosul is the largest city held by the militants, and has been one of the pillars of anti-government insurgency since the US-led invasion of Iraq in 2003.
Analysts believe that the fight will be much tougher with IS building-up its defences and preparing booby traps over the past months.
The tight streets and confines will also make it likely that west Mosul will be the site of major urban warfare in the coming months with expectations that civilian casualties and losses to Iraqi forces will be significant.
Around 750,000 civilians are believed to be trapped in west Mosul.
"IS resistance could be greater in this area and it will be harder, but all the more important, to completely clear the networks from Mosul after its recapture," Emily Anagnostos, Iraq analyst at the Institute for the Study of War told AFP.
Another analyst told the agency that the narrow streets should make it difficult or impossible for Iraqi armour to pass through leaving infantry vulnerable to ambush.
"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 area also holds symbolic value housing the mosque which IS leader Abu Bakr al-Baghdadi announced himself as caliph.
It is unclear what the next stage of the operation will be, but analysts believe Iraqi forces led by crack counter-terrorism units will first attack Mosul airport via pontoon bridges across the Tigris.