Prime Minister Haider al Abadi announced on television that the offensive was under way to "liberate the people from the terror of Daesh (IS)".

Coalition aircraft have already bombed jihadist positions near the city's IS-held airport, with Iraqi police and government forces expected to try to retake the facility.

Further south, troops have already seized two villages - Athbah and Al Lazzagah - as they advance towards the airport.

The country's second largest city is roughly split in half by the Tigris river, with government forces winning back control of the east in January following months of fighting.

The battle for the western part is set to be prolonged and difficult, due to more narrow streets in the old city which includes the mosque where IS leader Abu Bakr al Baghdadi proclaimed a "caliphate" in 2014.

Commander of the US-led coalition forces, Lieutenant General Stephen Townsend, warned: "Mosul would be a tough fight for any army in the world."

Paramilitary and Kurdish forces are also set to take on IS.

Many military vehicles will not be able to get through the narrow streets, forcing government troops to engage the enemy in close quarters.

"West Mosul has the potential certainly of being more difficult, with house-to-house fighting on a larger and more bloody scale," said intelligence analyst Patrick Skinner.

About 750,000 civilians are thought to be trapped in the west, and amid concerns for their safety the PM has told his forces to "respect human rights".

Aid organisations had feared an unprecedented exodus before the start of the fighting in the east four months ago but a significant majority of residents stayed at home.

Residents in the west have reported very difficult living conditions and warned they are already low on food, with weeks of fighting expected to lie ahead.
Bridges across the Tigris have now been destroyed by IS and the jihadists in the west are all but besieged.