Iraq's defense minister says the Sunni province of Anbar remains key to launching the long-awaited operation to retake Mosul, the country's second-largest city, from the Islamic State group.

Khalid al-Obeidi told reporters on Wednesday that before Iraqi forces can even plan the Mosul operation, they must "secure" Anbar so that it cannot serve as staging ground for militant counterattacks.

Anbar, west of Baghdad, was the first province to fall under Islamic State's control during the group's blitz last year.

Al-Obeidi's remarks reflected the challenges Iraqi forces, backed by U.S.-led coalition airstrikes, face as they try and claw back territory captured by IS.

At the moment, Iran-backed Iraqi soldiers and allied Shiite militias are fighting to retake Islamic State-held Tikrit, an offensive that is taking place without U.S. airstrikes.