Iraqi special forces have begun an assault on Mosul in the continuing operation to drive the Islamic State group from the country's second city.

Troops opened up with artillery, tank and machine gun fire on IS positions on the edge of the Gogjali neighbourhood, with the extremists responding occasionally with rocket-propelled grenades.

From the nearby village of Bazwaya, smoke could be seen rising from buildings on the city's edge, where shells were landing.

Brigadier General Haider Fadhil of the special forces said the operation began on Tuesday morning.

For more than two weeks, Iraqi forces and their Kurdish allies, Sunni tribesmen and Shiite militias have been converging on Mosul from all directions to oust IS from the city.

The operation is expected to take weeks, if not months.

US-led coalition jets joined the attack on Gogjali with air strikes.

If the forces enter the neighbourhood, it will be the first time Iraqi troops set foot in Mosul in more than two years, after they were driven out by a much smaller force of IS extremists in 2014.