Iraqi forces have started their assault on Mosul's Old City as they press their battle to retake the northern city from the so-called Islamic State group.

Taking back the Old City, a densely populated area of narrow alleyways on the western side of Mosul, is crucial to recapturing the whole of the former IS bastion.

The United Nations said around 100,000 civilians are trapped in the ancient city centre.

"The army, counter-terrorism forces and federal police launched an attack on the Old City," Staff Lieutenant General Abdulamir Yarallah said in a statement.

Iraqi forces backed by the air strikes of the US-led coalition are pressing a months-long offensive to retake the Old City on the west side of the city from the jihadists.

"The initial air strikes started at around midnight. The security forces started storming parts of the Old City at dawn," an officer with Nineveh operations command said.

Iraqi forces launched the battle for Mosul in October, retaking the eastern part of the city in January and starting the operation for its western part the next month.

The so-called Islamic State group overran Mosul in 2014, declaring a self-styled "caliphate" over areas it ruled.