A convoy of US forces armoured vehicles drives near the village of Yalanli, on the western outskirts of the northern Syrian city of Manbij, on March 5, 2017.

Syrian forces backed by the United States are struggling to contain ISIS following President Donald Trump's partial withdrawal of U.S. troops from the country, a Defense Department watchdog warned in a report released this week.

The report, produced by the department's office of inspector general, found that between April and June the so-called Islamic State consolidated its power in Iraq and mounted a resurgence in Syria.

Trump announced in December that the U.S. would soon reduce its fighting force in Syria, surprising allies and prompting the departure of then-Defense Secretary James Mattis.

"We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency," he wrote in a post on Twitter announcing the decision.

"I defeated 100% [of the] caliphate," Trump said during an interview with C-SPAN last month. "That doesn't mean ISIS doesn't go running around bombing a store and doing what they do, because they're stone cold crazy, but we've done a great job."

But the report, which largely summarizes the views of the U.S.-led military task force overseeing operations against ISIS, says that the group remains a threat in Iraq and Syria despite the loss of its final stronghold in Syria earlier this year.

The report says that ISIS retains between 14,000 and 18,000 members in Iraq and Syria, including about 3,000 foreign fighters. It also retains an extensive, global social media operation to recruit fighters.

In both countries, ISIS fighters carried out targeted assassinations, ambushes, suicide bombings in public places, and set fire to fields of crops, the report said.

The White House did not respond to a request for comment.

The insurgency did not carry out conventional attacks or attempt to take back territory, the report said, instead it sought to wreak havoc and decrease support for government institutions.

In Syria, it also began to reestablish "illicit fundraising capabilities" which the report likened to "taxes" on residents and businesses where it operates, the inspector general found. In Iraq, it was able to establish logistics nodes for coordinating attacks, bolstered by fighters who arrived from Syria following their final territorial defeat in March.

The group is strongest in Syria in the Dayr az Zawr province, parts of Raqqah province, and Homs province near Palmyra, the report said.

The report says the U.S. drawdown, completed in the spring, left American-backed Syrian forces ill-prepared to develop trust with local communities and develop human-based intelligence "necessary to confront ISIS resurgent cells and insurgent capabilities in Syria."

"Despite losing its territorial 'caliphate,' the Islamic State in Iraq and Syria (ISIS) solidified its insurgent capabilities in Iraq and was resurging in Syria this quarter," the report says.

In one massive camp in Syria, Al Hol, the absence of U.S. troops has left Syrian partners without the ability to provide more than minimal security, leading to the "uncontested conditions to spread of ISIS ideology."

"ISIS is likely exploiting the lack of security to enlist new members and re-engage members who have left the battlefield," the report says.

The withdrawal also limited the U.S. capacity to track the humanitarian situation at the camp, which is home to 70,000 people, two-thirds of which the International Committee of the Red Cross estimates are children.

The United Nations estimates that 11,000 residents of the camp are relatives of ISIS fighters, and the U.S. military estimates that as many as 45,000 are ISIS supporters.

In the coming months, the U.N. estimates that upward of 30,000 Iraqi residents at al Hol, primarily women and children, will return to Iraq.

Neither the Department of Defense nor the Department of State responded to a request for comment.

Secretary of State Mike Pompeo said Wednesday that he had not read the report, The Wall Street Journal reported, but noted that the administration "is incredibly mindful of the success we've had versus ISIS, and the challenge that it continues to present to the world."