Archive picture of an Islamic State fighter.
The so-called Islamic State (IS) is "re-surging" in Syria, a new Pentagon inspector general's report said.
The news comes less than five months after US President Donald Trump declared the terror group's caliphate in Syria "100% defeated".
The report, published on Tuesday, warns that, despite losing its territorial caliphate, IS has "solidified its insurgent capabilities in Iraq" and was "re-surging in Syria".
The report, from the lead inspector general for Operation Inherent Resolve, the official name for the US-led operation fighting ISIS, covers the period 1 April to 30 June of this year.
"The reduction of US forces has decreased the support available for Syrian partner forces at a time when their forces need more training and equipping to respond to the ISIS resurgence," the Principal Deputy Inspector General wrote in a message accompanying the report.
The report warns that, because of the reduction in personnel, the US and its local allies were unable to closely monitor the al-Hol internally displaced persons camp.
The situation, according to the report, has allowed "ISIS ideology to spread 'uncontested' in the camp", potentially allowing IS to replenish its ranks among the tens of thousands of inhabitants.