US Secretary of Defense Ash Carter said Sunday that operations to retake the Iraqi city of Mosul from the Islamic State group were key to defeating the jihadist group.

“This is a decisive moment in the campaign to deliver ISIL a lasting defeat,” Carter said in a statement.

“We are confident our Iraqi partners will prevail against our common enemy and free Mosul and the rest of Iraq from ISIL`s hatred and brutality.”

Carter promised continued support for Iraq.

“The United States and the rest of the international coalition stand ready to support Iraqi Security Forces, Peshmerga fighters and the people of Iraq in the difficult fight ahead,” he stressed.

The northern city was where IS supremo Abu Bakr al-Baghdadi publicly proclaimed a “caliphate” straddling Iraq and Syria in June 2014.

With the support of Iran and a US-led coalition, Iraqi forces have since regained much of the ground lost to IS and Mosul is the extremist group`s last major stronghold in Iraq.

Prime Minister Haider al-Abadi has said only government forces will enter Mosul, a Sunni-majority city that IS seized with relative ease partly amid local resentment towards the Shiite-dominated security forces.

Shiite militia groups have been accused of serious abuses against Sunni civilians in the course of operations to reconquer territory from IS.