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The final push to retake Hit is being led by the elite counter-terrorism service and involves a range of ranking officers including army soldiers, police and local tribal fighters.

"They have begun a broad operation to liberate Hit and Kubaysa," Major General Ali Ibrahim Daboun, the head of the Al-Jazeera Operations Command, told AFP.

The two cities, 90miles (145km) from the capital, have been under IS control since the militants overran the area in October 2014.

But pro-government forces retook parts of the cities, placing a symbolic Iraqi flag where militants once raised their black standard.

"Members of the terrorist [IS] gangs have fled back into the town centre," the head of the local council for Al-Baghdadi district, Malallah al-Obeidi, told AFP. 

US-led coalition jets are providing ground forces with logistical air support in the final move towards the militants.

But the fate of thousands of residents is on the line in the latest mission.

Aid agencies, including the Red Cross, voiced concern for the estimated 35,000 forced to flee from Hit in preparation for the battle.

The humanitarian group, which has delivered aid for the first time to an estimated 12,000 people outside Hit, said thousands more were thought to be stranded in remote areas with little assistance.

"We don't know how they managed to survive. Repeated access is crucial in order to help the remaining thousands of people who urgently need humanitarian aid," said Katharina Ritz, head of the ICRC delegation in Iraq.

IS still controls much of northern and western Iraq, but has been driven back in recent months in some areas, such as the cities of Ramadi and Tikrit.

The government last month declared the western city of Ramadi, the Anbar provincial capital, "fully liberated" after it was captured by IS last year.

A campaign to retake Mosul, the main city held by Islamic State group in Iraq, has long been believed to be imminent -  but has not yet taken off the ground.