An Iraqi offensive to take back Mosul from ISIL gained new momentum with an armoured division trying to advance into the city from the northern side.

The Iraqi army's 9th Armoured Division and Rapid Response units of the interior ministry opened a new front on Thursday in the northwest of the city against Islamic State of Iraq and the Levant, according to the military.

The armed group is now besieged in the northwestern corner of Mosul, which includes the historic Old City centre and the medieval Grand al-Nuri Mosque and its landmark leaning minaret, where their black flag has been flying since June 2014.

"Our forces are making a steady advance in the first hours of the offensive and Daesh [ISIL] fighters are breaking and retreating," Brigadier-General Yahya Rasool, a spokesman for the joint operations command, told state television.

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Iraq's Joint Operations Command said in a statement the targets of the latest push were northwestern neighbourhoods on the edges of Mosul called Musharifah, Kanisah and al-Haramat.

"Now your sons are fighting and striking the enemy's defences... They rejoice in victory or martyrdom for the sake of liberating the rest of the city of Mosul from Daesh [ISIL] terrorists," it said.

On the southern edge of Mosul's west, Iraq's federal police are just a few hundred metres from Mosul's al-Nuri mosque.

The mosque is a hugely symbolic prize as it is where ISIL leader Abu Bakr al-Baghdadi made a rare public appearance in July 2014 and declared the group's self-styled caliphate after the armed group seized almost one-third of Iraq.

According to the authorities, ISIL now controls less than seven percent of Iraq.

The mosque lies inside Mosul's old city where some of the toughest fighting is expected to unfold because of the narrow streets. Iraqi forces now appear to be moving to surround the Old City before launching an operation to clear it.

Al Jazeera's Mohammed Jamjoom, reporting from Erbil, said capturing the area could be key to securing all of Iraq's second-largest city.

"If the armed forces manage to retake these [neighbourhoods], then the advance of other units from the south will be easier. The final aim is to retake the area where al-Baghdadi proclaimed the caliphate," said Jamjoom.

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The fight for Mosul's west has been slow and deadly to Iraqi forces and civilians caught in the crossfire.

More than six months of fighting has killed and wounded more than 8,000 civilians, according to the United Nations, a number that only counts people transferred to hospitals from front-line clinics.

Hundreds of thousands of people are still trapped inside the Mosul neighbourhoods under ISIL control and some 419,000 people have been forced to flee western Mosul alone since the fighting began there, according to a UN statement released this week.