Washington (CNN) ISIS will claim victory over the US and is likely to regain territory as a result of President Donald Trump's decision to withdraw from Syria, according to a new report released Monday by the Pentagon's Inspector General.

"ISIS may conduct opportunistic attacks on US personnel as they withdraw but will leverage the event as a 'victory' in its media," the report said, citing information obtained from US Central Command, which oversees US military operations in the region.

The report, the first of its kind since Trump announced plans to pull all troops from Syria, also says "ISIS remains an active insurgent group in both Iraq and Syria."

"ISIS is regenerating key functions and capabilities more quickly in Iraq than in Syria, but absent sustained [counterterrorism] pressure, ISIS could likely resurge in Syria within six to twelve months and regain limited territory," the report adds.

However, a US official told CNN last week that some military and intelligence officials are concerned an ISIS resurgence could happen even sooner without the US counterterrorism presence.

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