WASHINGTON — U.S.-backed forces in Iraq and Syria are struggling to contain the ISIS insurgency, according to a Pentagon report released Tuesday.

Between April and June 2019, the Islamic State terror group "solidified its insurgent capabilities in Iraq and was resurging in Syria," the Department of Defense Inspector General Quarterly Report says.

The report references significant changes in the region. In addition to the president’s sudden announcement of a drawdown of U.S. troops in Syria -– against the counsel of military advisers -– the report raises concerns that ISIS is gaining a renewed foothold in Iraq because U.S. personnel there were evacuated from diplomatic posts due to rising tensions with Iran. Experts told NBC News that as a result, that also has led to less surveillance of ISIS activity in Iraq.

The State Department said that the departure of diplomatic staff from Baghdad and Erbil "hindered U.S. stabilization efforts in Iraq," according to the report. It also notes that the troop withdrawal from Syria came "at a time when U.S. commanders said the [Syrian Defense Forces] needed more training and equipping for counterinsurgency operations."

The report noted that the withdrawal also reduced the ability of U.S. forces to maintain a presence at a sprawling refugee camp where tens of thousands of displaced people from former ISIS territory are living. With Syrian troops only able to provide "minimal security" at the camp, it has "created conditions that allow ISIS ideology to spread 'uncontested."

While ISIS has not regained territory, the Pentagon watchdog says the terror group has up to 18,000 active fighters and has launched "targeted assassinations, ambushes, [and] suicide bombings" since April.

The assessment contrasts with Trump's repeated declarations of victory over ISIS.

"We have 100 percent of the caliphate and we're rapidly pulling out of Syria," Trump said at a July 16 Cabinet meeting. "We'll be out of there pretty soon, and let them handle their own problems."

Last January, NBC News exclusively obtained a draft of the previous Pentagon report that warned ISIS could regain territory in six to 12 months in the absence of sustained military pressure. The new report builds on that warning.

Former Defense Secretary Jim Mattis warned of the same possibility when he quit last December.