Iraqi forces on Sunday started their assault on Mosul's Old City as they press their battle to retake the northern city from the Islamic State group, an army commander said.

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

Staff Lieutenant General Abdulwahab al-Saadi, a senior commander with the Counter-Terrorism Service, confirmed the "start of the assault on the Old City".

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.

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

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

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

Source: AFP