The draft report will be delivered to Pentagon officials and lawmakers on Friday, with the full report due to be made public Monday. NBC News first reported on the draft's conclusion.
The inspector general's office releases quarterly reports about the progress of the anti-ISIS campaign in Iraq and Syria, dubbed Operation Inherent Resolve. The office's previous quarterly report, made public in November, made headlines when it estimated that between 20,000 and 30,000 ISIS fighters were still in Iraq and Syria.
The same report said that ISIS had lost all of the territory it had held in Iraq and all but 1 percent of the territory it once held in Syria. However, it added that U.S. military officials and United Nations monitors said that militants had gone underground and were waging an "effective clandestine" insurgency in both countries.
"Thousands of ISIS fighters remain in Syria. In the near term, we expect they will focus on guerrilla-type tactics after losing the remaining ground under ISIS control," a second senior U.S. defense official told Fox News. "I believe it’s fair to say that they could retake some territory if they are not subject to pressure. We are working on ways to sustain pressure after U.S. ground forces have withdrawn."