Duterte took the stage of the Philippines-India Business Forum in New Delhi on Friday.
The head of the Philippine military said only about 30 militants remained in the formerly ISIS-held city and that victory would come "in a matter of days."
The news comes as a surprise to multiple Filipino officials, who say they have no knowledge of such an operation.
The Philippines military said child soldiers are among the fighters left in the embattled city.
The Maute Group and other rebels are targeting Christians in the fight to takeover the city.
The army has used airstrikes and street combat, yet still the Maute Group Islamist militants fight on.