Iraqi forces face a massive challenge as they press on to retake western Mosul, but the days of the Islamic State group are counted, a UN envoy said Thursday.

Iraqi Prime Minister Haider al-Abadi declared on January 24 that his forces had retaken east Mosul and the battle was now moving to the other side of the Tigris River.

"This steady progress should not conceal that fighting has been and will be a massive challenge, in particular inside the old city in western Mosul," UN envoy Jan Kubis told the Security Council.

"Yet in the rather short foreseeable future, the liberation operations in Iraq are coming to an end - the days of the so-called ISIL are counted."

Iraq launched an offensive in October to drive out IS fighters who seized Mosul in 2014.

Kubis voiced concern over the "extremely high percentage" of civilian casualties, adding there was "clear evidence" from gunshot wounds that civilians were being targeted by IS combatants.

The envoy added that civilians will be at "extreme risk" when the fighting starts in the western sections of Mosul.