Iraqi Prime Minister Haider al-Abadi announced the start of an offensive to retake western Mosul from the so-called "Islamic State" (IS) militant group, months into a grueling campaign to recapture all of the city.

"We announce the start of a new phase in the operation, we are coming Nineveh to liberate the western side of Mosul," Abadi said in a televised speech, referring to the province of which Mosul is the capital.

"Our forces are beginning the liberation of the citizens from the terror of Daesh," Abadi said, refering to IS using an Arabic acronym for the terror group. 

A top commander said later on Sunday that Iraqi forces had retaken two villages south of the city during their push on the city's west bank.

Fierce fighting

Iraqi forces began a large-scale offensive to retake the northern city from the jihadis on October 17 last year, securing the east bank last month.

Observers say that retaking the west bank is likely to be even more difficult, as the narrow streets of the Old City located there will be impassable for many military vehicles. This means that Iraqi government forces will often have to fight the jihadis on foot.

Fighting in the city has been impeded by the presence of some half a million civilian residents - a factor that has possibly averted an even more serious humanitarian crisis, as the most deadly weaponry has not been deployed by either side.

Civilians at risk

However, residents of western Mosul say they are living under very difficult conditions and are suffering from food scarcity, with weeks of fighting still likely to come.

Following Sunday's announcement of the assault on the west bank, the United Nations warned that civilians were "at extreme risk."

"The situation is distressing. People, right now, are in trouble," Lise Grande, Humanitarian Coordinator for Iraq, said in a statement.

"We are hearing reports of parents struggling to feed their children and to heat their homes," the statement said.

Last major IS bastion in Iraq

Mosul has been in the hands of IS since June 2014, when the extremist group overran swathes of territory north and west of Baghdad in a lightning offensive. At the time, its leader, Abu Bakr al-Baghdadi, made a public appearance in the city in which he proclaimed a "caliphate" across the territory IS had captured in Iraq and Syria. 

The city is now the last major IS stronghold in Iraq, and the extremist group has put up stiff resistance to defend it.

tj/rc (AP, AFP)