Iraq's prime minister has announced the launch of a military assault aimed at taking western Mosul from the Islamic State of Iraq and the Levant (ISIL) group.

The Iraqi forces seized 17 villages from ISIL, also known as ISIS, on Sunday, according to top army commander Abdul Ameer Yarallah, advancing from several directions towards Mosul airport, which lies just south of the city.

"We announce the start of a new phase in the operation. We are coming to Nineveh to liberate the western side of Mosul," Haider al-Abadi said on television.

A top army commander later announced that forces led by federal police units retook villages south of Mosul, including Athbah, which leaves them within striking distance of the airport.

"We launched our operation at 7am (04:00 GMT) ... We are heading towards the airport," said Abbas al-Juburi of the interior ministry's elite Rapid Response force.

The sky south of Mosul was black with smoke from air strikes and artillery as thousands of forces in armoured convoys worked converged on the airport.

"They're desperate," Ali, a Rapid Response officer, said in the village of Al-Buseif as helicopters flew overhead, tracking the last ISIL fighters attempting to flee.

"They will try to cause as many losses as possible, because they know they will die anyway," his colleague Alaa said.

Al Jazeera's Osama Bin Javaid, reporting from Erbil in northern Iraq on Sunday, said several groups were involved in a complex assault on ISIL positions.

"[Mosul is] flanked from the eastern side by counterterrorism forces, from the south by the Iraqi police, and from the north by the Iraqi military and the popular mobilisation forces, also known as the Shia militias," he said.

He said about 750,000 civilians are still believed to be trapped in the city's west.

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Aid organisations had feared an exodus of unprecedented proportions before the start of the fighting, which began four months ago with a government push on the east, but a significant majority of residents stayed home.

The aid community fears a bigger exodus from west Mosul, however.

"We are racing against the clock to prepare emergency sites south of Mosul to receive displaced families," Lise Grande, the UN's humanitarian coordinator in Iraq, said in a statement.

Save the Children urged all parties to protect the estimated 350,000 children currently trapped in west Mosul.

"This is the grim choice for children in western Mosul right now: bombs, crossfire and hunger if they stay - or execution and snipers if they try to run," said Maurizio Crivallero, the charity's Iraq director.

The Iraqi air force had on Saturday dropped leaflets on the west of the city, warning that a ground attack to push ISIL fighters out was imminent.

"Get ready to welcome the sons of your armed forces and to cooperate with them, as your brothers on the left side have done, in order to reduce losses and speed up the conclusion [of the battle]," read one leaflet.

Other leaflets warned ISIL fighters to "lay down their weapons and surrender".

The defence ministry announced last month that ISIL had been almost completely pushed out of the eastern side of Mosul, which is divided by the Tigris.

It later retracted that statement and fighting continues in some parts of the east.

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More than 46,000 people who fled Mosul as fighting raged have, though, been able to return to its eastern districts over the past few weeks despite the pockets of fighting.

Though the west bank of Mosul is slightly smaller than the east, the battle there is expected to be more difficult for government forces, especially in the narrow streets of the Old City.

The streets around the historic centre, which includes the mosque in which ISIL leader Abu Bakr al-Baghdadi made his only public appearance in June 2014, will be impassable for many military vehicles and force government fighters to take on ISIL in direct fighting.

The major push to recapture Mosul was launched on October 17, supported by bombing raids from a US-led coalition.