Pro-government Iraqi tribal forces hold a position in the Tel Mushaihed area east of Ramadi, a large city on the Euphrates River 60 miles west of Baghdad, Dec. 16, 2015. AFP/Getty Images

Iraqi warplanes dropped leaflets Sunday on the city of Ramadi, controlled by the Islamic State group, requesting residents to leaving within 72 hours, Reuters reported, citing a state television account of a military statement. Located about 60 miles west of Baghdad, Ramadi fell into the hands of the militant group also known as either ISIL or ISIS about six months ago.

An unidentified military official told Reuters the move was "an indication that a major military operation to retake the city center will start soon." There was no more information available about the situation early Sunday.

Last week, Iraqi security forces said they had made progress on two fronts in Ramadi, clearing Islamic State group militants out of a key military command base and a neighborhood in the far west area of the city. Iraqi intelligence estimated that between 250 and 300 militants remain entrenched in the center of the city, Reuters reported. The campaign to retake the city could be a tough urban battle, with the militants' defense encompassing booby-trapped buildings, elaborate tunnels and roadside bombs, the New York Times reported this month.

Iraq's military indicated early this month it was prepared to carry out a final assault against the Islamic State group in the western city, the Telegraph reported. The loss of Ramadi to the militant group in May was a major blow to the Iraqi government amid the U.S.-led campaign to drive the terrorist group out of the country.

The Iraqi military dropped leaflets in early December as well, warning residents still in the city to leave through a southern exit, the Telegraph said. The Islamic State group responded by setting up checkpoints to preventing civilians from leaving, according to the Associated Press. Residents told the news service that the militant group also threatened residents.

"Loudspeakers from mosques give warnings that civilians are not allowed to leave, and anyone who tries to do so will be either arrested or killed," AP quoted one resident as saying on the condition of anonymity.

At the beginning of the month, it was estimated that from 4,000 to 10,000 residents remained in the city, AP reported.