Pro-Baghdad troops started storming the historic Mosul quarter at dawn Sunday, in what is expected to be the final stage of a months-long battle to oust IS from their last urban stronghold in Iraq.

Special forces commander Abdel Ghani al-Asadi told Iraqi state TV he expected a "vicious and tough fight."

"This is the last chapter" in the offensive against the extremists, he added.

Mosul has been controlled by IS since 2014, with the US-backed Iraqi forces launching a battle to retake it last October. The assault has since managed to push the militia out from eastern parts of the city, but they still control the crowded Old City.

Up to 150,000 people were trapped in the town quarter in "desperate" conditions, with little food and no clean water, UN humanitarian coordinator Lise Grande told the Associated Press news agency. 

Some 862,000 people have been displaced from Mosul over the last eight months, although 195,000 have since returned, mainly to the liberated east of the city.

At the height of its power, the group controlled vast swathes of land from Iraq's second city, which served as the militia's capital in the war-torn country. It has since lost most of its territory and was forced out of all Iraqi cities except Mosul. The militia still controls Raqqa in Syria.

dj/jlw (AP, AFP, Reuters, dpa)