The Iraqi army said Thursday its troops and allied militia had launched what is expected to be a long and difficult offensive to retake the second city of Mosul, the "Islamic State" (ISIS) terrorist group's main hub in Iraq, AFP reports.

The army and the Popular Mobilisation paramilitary force "have begun the first phase of conquest operations" in the northern Nineveh province, of which Mosul is the capital, Iraq's joint operations command said in a statement.

It said four villages had been taken between the town of Qayyarah, which is still held by ISIS, and Makhmur, where US-backed Iraqi forces have been massing in recent weeks. The army did not say how long this phase of the operation was expected to take and Iraqi forces still look far from being in a position to take the city itself.