A general view of the Tigris River is seen from the compound of the closed Nineveh International Hotel in Mosul on June 16, 2017 which was recovered by Iraqi troops from Islamic State group fighters earlier in the year (AFP)
Iraqi forces launched an assault Sunday to retake Mosul's Old City, the last district still held by the Islamic State (IS) militant group three years after IS seized the northern city and declared their "caliphate".
Military commanders told AFP the assault had begun at dawn, after overnight air strikes by the US-led coalition backing Iraqi forces, and that IS were putting up fierce resistance.
The push into Mosul's Old City -- a densely populated warren of narrow alleyways on the western side of Mosul -- marks the culmination of a months-long campaign by Iraq forces to retake IS's last major urban stronghold in the country.
The loss of Mosul would mark the effective end of the Iraqi portion of the cross-border "caliphate" that IS declared in the summer of 2014, after seizing control of large parts of Iraq and neighbouring Syria.
A military statement announced the start of the assault, saying the army, counter-terrorism forces and federal police "launched an attack on the Old City".
Machinegun fire crackled and plumes of smoke from missiles rose above the Old City on Sunday morning.
Staff Lieutenant General Abdulghani al-Assadi, a senior commander with the Counter-Terrorism Service, said the operation was advancing slowly "to preserve civilian lives as we breach the enemy's defence lines".
"Our forces have moved in on foot because the alleys are very narrow," he said. "The strategy has changed compared to other operations. There is no room for our vehicles to manoeuvre and there are many civilians."
Surrounded by Iraqi forces on three sides and blocked on the other by the Tigris River that runs through Mosul, IS had no choice but a fight to the finish, he said.
"This is the last episode of the Daesh show," Assadi said, using an Arabic acronym for IS.
"It's our most difficult operation. Fighting is fierce because it's their last stronghold... They have nowhere to flee."
He said he hoped the operation could be concluded before Eid -- the festival marking the end of the Muslim holy month of Ramadan, expected around June 25 or 26 -- "but I think it is going to take longer".
Iraqi forces launched the battle for Mosul -- the country's second-largest city -- in October, retaking the eastern part of the city in January and starting the operation for its western part the next month.
The United Nations said Friday that IS may be holding more than 100,000 civilians as human shields in the Old City.
The UN refugee agency's representative in Iraq Bruno Geddo said IS had been capturing civilians and forcing them into the Old City.
"We know that ISIS moved them with them as they left... locations where the fighting was going on," he told reporters in Geneva, using another acronym for IS.
"These civilians are basically held as human shields in the Old City."
With virtually no food, water or electricity left in the area, civilians are "living in an increasingly worsening situation of penury and panic", he said.
Since the battle to retake Mosul began, an estimated 862,000 people have been displaced from the city, although 195,000 have since returned, mainly to its eastern side.
It was in Mosul in July 2014 that IS chief Abu Bakr al-Baghdadi made his only public appearance, to urge Muslims worldwide to move to the recently proclaimed "caliphate" straddling Iraq and Syria.
IS has since lost most of the territory they once controlled in the face of US-backed offensives in Iraq and in Syria, where an alliance of Kurdish and Arab forces are advancing on their last Syrian bastion Raqa.
It was not clear how many IS fighters remained in Mosul, where many foreign fighters have joined IS since the city was taken.
"The locals in Daesh will shave their beards and try to blend in with the civilians as they always do," Assadi of the CTS said. "The foreigners will fight hard and eventually get killed."
The fall of Mosul was the worst defeat that Iraqi forces suffered in the war with IS, and regaining it would cap a major turnaround for security forces, who broke and ran despite outnumbering IS militants who attacked the city in 2014.