A forthcoming report from the Pentagon's inspector general says the Islamic State (ISIS) terrorist group will regain territory it has lost in Syria in a matter of months without military pressure, a senior U.S. defense official told Fox News late Thursday.

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.

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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 We are working on ways to sustain pressure after U.S. ground forces have withdrawn."

President Trump announced plans for a pullout of U.S. troops from Syria back in December, asserting that ISIS had been defeated. The announcement prompted the resignation of Defense Secretary Jim Mattis.

Secretary of State Mike Pompeo told Fox News' "Hannity" on Wednesday that Trump would make a "significant announcement" in next week's State of the Union address "with respect of the status of the [ISIS] caliphate."

"In spite of the enormous progress we've made and the success that we've had ... the threat from radical Islamic terrorism is real and we need to continue to do all that we can to make sure that there's not a resurgence of ISIS or that all the other variants of that terror regime continue to be under pressure," Pompeo said. "This administration is committed to that."

On Thursday, the Senate voted overwhelmingly to advance a nonbinding amendment from Senate Majority Leader Mitch McConnell, R-Ky., opposing withdrawal from Syria in addition to a Trump plan to remove up to half of all American forces from Afghanistan.

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The measure warns that Islamic militants still pose a serious threat to the U.S., adding that a "precipitous withdrawal" of American forces from those countries could "allow terrorists to regroup, destabilize critical regions and create vacuums that could be filled by Iran or Russia."

"ISIS and al-Qaida have yet to be defeated," McConnell told reporters. "And American national security interests require continued commitment to our missions there."

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The majority leader's amendment would encourage cooperation between the White House and Congress to develop long-term strategies in both nations, "including a thorough accounting of the risks of withdrawing too hastily." A vote on final passage could come early next week.

If the amendment succeeds, it will be added to a wide-ranging foreign policy bill that has been pending on the Senate floor for several weeks. The legislation includes measures supporting Israel and Jordan and slapping sanctions on Syrians involved in war crimes.

The Associated Press contributed to this report.