The offensive to liberate Mosul from the control of Islamic State (IS or ISIS) has begun, Iraqi Prime Minister Haider Al-Abadi said in a televised statement early Monday.

"Our dearest people in Nineveh province, the victory bell has rung, and the operations to liberate Mosul have begun," he said. "I am announcing today the beginning of these heroic operations to liberate you from the brutality and terrorism of ISIS. God willing, we will meet soon on the ground of Mosul where we will all celebrate the liberation and your freedom."

Only forces with the Iraqi army and National Police will enter the city "and no others," Al-Abadi was quoted as saying by CNN.

The assault from Kurdish Peshmerga, Iraqi government and allied forces is backed by the US-led coalition fighting IS in Iraq.

Mosul, Iraq's second largest city, has been under IS control since June 2014. The UN has warned that the humanitarian impact could be "enormous", and affect up to 1.2 million people, reported BBC.

Mosul is the group's last major stronghold in Iraq. The loss of the city, officials say, would mark the effective defeat of IS in the country.

Sporadic artillery was heard in areas around the city after the announcement, AP news agency reported.