The U.S. military believes that "absent sustained pressure" on the Islamic State, ISIS could re-emerge in Syria within six to 12 months, according to a new Department of Defense Inspector General report on Operation Inherent Resolve.

According to the Pentagon, while U.S.-backed Syrian forces have continued the fight to retake the remaining ISIS strongholds in Syria, ISIS remains a "potent force of battle-hardened and well-disciplined fighters that could likely resurge in Syria absent continued counterterrorism pressure," the report reads.

Read the report here:

The report notes that currently ISIS is "regenerating key functions and capabilities more quickly in Iraq than Syria." ISIS also retains the ability to coordinate offensives and counteroffensives. And it is able to operate despite its lost territory, as a "decentralized insurgency," the report says.

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The administration policy is to maintain pressure on ISIS, even with the withdrawal of American troops. President Trump announced in December that the military would withdraw from Syria, but according to the Defense Department, approximately 2,000 ISIS fighters remained where U.S. forced had previously been operating.

ISIS fighters in that region were described by operation officials as "battle-experienced, hardened and well disciplined."

The report echoes what intelligence chiefs told lawmakers on Capitol Hill last week -- that a vacuum left by the departure of U.S. troops could be filled by ISIS or al Qaeda in Syria or Iraq.

Mr. Trump, however, told CBS' "Face the Nation" that if the groups gain power, the U.S. could "come back if we have to."

"We have very fast airplanes, we have very good cargo planes. We can come back very quickly, and I'm not leaving. We have a base in Iraq and the base is a fantastic edifice. I mean -- I was there recently, and I couldn't believe the money that was spent on these massive runways. And these -- I've rarely seen anything like it. And it's there. And we'll be there," he said.

He went on to say he'd keep troops in Iraq to be able to "watch Iran," which elicited a strong reaction from Iraqi President Barham Salih.

According to the Associated Press, Salih said in remarks in Baghdad on Monday that he found Mr. Trump's comments "strange," considering U.S. troops were in Iraq as part of an agreement between the two countries and with a specific mission of assisting in the fight against ISIS. He said the Iraqi constitution forbids the use of Iraq as a base to threaten the interests or security of neighboring countries.

Mr. Trump told Brennan that 2,000 troops remain in Syria, but they are starting to come home, as they push out the "final remainder of the caliphate." Afterward, he said, "they will be going to our base in Iraq, and ultimately, some will be coming home."

CBS News' David Martin contributed to this report.