"The available data indicate that Iraq requires three months to eliminate Daesh," Haider al-Abadi told a televised news conference, referring to the jihadist group by an Arabic acronym.
Previously, he had vowed that the IS-occupied city of Mosul would be retaken "before the end of the year", a goal that is no longer possible.
What began as a rapid push into Mosul, which the jihadists have occupied since June 2014, has turned into a hellish block-by-block war, with IS inflicting high casualty rates on advancing Iraqi forces.
High-ranking officials believe that the battle against IS could drag on, and the Sunni extremist group continues to carry out attacks in areas from which it had been dislodged by the government offensive.
Last week, an IS triple car bombing on a market in Gogjali a few kilometres east of Mosul killed at least 23 people.
The army had retaken Gogjali from the jihadists in early November after more than two years of IS occupation.
The jihadist group overran large areas north and west of Baghdad in 2014, but Iraqi forces backed by US-led air strikes have since regained much of the territory originally lost to IS.
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