He was speaking during a visit to Karbala, a city holy to Shiite Muslims, where millions of pilgrims gathered Friday to mark the annual Arbaeen commemoration.
"The damage caused by the IS occupation of Iraqi cities already amounts to more than USD 100 billion," Abadi said.
"That's just the damage to the economy and infrastructure."
IS, a radical Sunni group, seized around a third of Iraq and parts of Syria in a sweeping 2014 advance.
Iraqi forces launched an operation Saturday to retake the last IS-held towns in Iraq, including the Euphrates valley town of Rawa and nearby villages.
The assault also aims to "clean open areas in the desert" of western Iraq, said Abadi, who is also head of the armed forces.
Arbaeen is one of the biggest religious festivals on earth and marks the end of the 40-day mourning period for the 7th-century killing of Imam Hussein by the forces of the Caliph Yazid -- a formative event in Shiite Islam.
The pilgrimage, under tight security, passed off without incident.
The expulsion of IS from Iraqi cities added an extra dimension to this year's pilgrimage, as the Sunni extremist group has repeatedly targeted Shiites.
Last year, a suicide bombing killed at least 70 mainly Iranian worshippers returning from the commemoration.
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