U.S. plans to keep just a residual force in Syria to ensure the enduring defeat of the Islamic State may be on the verge of backfiring, with some military officials warning the strategy is giving the terror group new life.

The doubts, raised in a Defense Department Inspector General report released Tuesday, come as Washington has struggled to secure additional on-the-ground help in Syria from allies and amid renewed warnings that while IS may have lost control of its self-declared caliphate, the group's fighters are far from defeated.

Some of the strongest criticism for what the report described as Washington's completed "partial withdrawal" from Syria is from officials with Combined Joint Task Force-Operation Inherent Resolve (CJTF-OIR), the headquarters responsible for overseeing U.S. and coalition efforts against IS.

"According to CJTF-OIR, the reduction of U.S. 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," Principal Deputy Inspector General Glenn Fine wrote, using another acronym for Islamic State.

FILE -- A member of U.S.-backed Syrian Democratic Forces (SDF) watches over people who were evacuated out of the last territory held by Islamic State militants, outside Baghuz, Syria, March 5, 2019.

Additionally, coalition officials told the inspector general the drawdown could cause U.S.-backed forces, including the mostly Kurdish Syrian Democratic Forces (SDF), to turn away from Washington and seek out "alternate partnerships and resources."

'Detrimental' to U.S. mission

CJTF-OIR warned that such developments could be "detrimental to the United States' mission in Iraq and Syria."

Neither the White House nor the Pentagon immediately responded to requests for comment on the report, which also identified other shortcomings with White House strategy and U.S. military efforts in Syria and Iraq.

U.S. President Donald Trump first announced the U.S. withdrawal from Syria this past December via Twitter.

We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency. — Donald J. Trump (@realDonaldTrump) December 19, 2018

But the announcement sparked significant divisions within the administration, especially from military officials, ultimately prompting then-Defense Secretary Jim Mattis to resign.

Differences With Trump's Views Prompted Mattis Departure US secretary of defense announced he was quitting, personally handing his letter of resignation to president following a lunch meeting at the White House

Later, the outgoing commander of U.S. military forces in the Middle East, General Joseph Votel, told lawmakers he was never even asked for advice.

"I was not consulted," he said during a congressional appearance this past February.

Top US General 'Not Consulted' About Syria Troop Withdrawal 'I was not aware of the specific announcement,' Gen. Joseph Votel told US lawmakers Tuesday of the president's announcement this past December

Despite concerns, Pentagon officials proceeded with what they described as a deliberate and orderly withdrawal of most of the 2,200 U.S. troops in Syria, completing the drawdown in the months following the fall of IS's last Syrian stronghold in late March.

But since then, there have been numerous warnings that while IS may no longer have outright control of territory, its grip on Syria remains strong.

IS' "covert network in the Syrian Arab Republic is spreading, and cells are being established at the provincial level," a United Nations report warned last week, adding the terror group "is adapting, consolidating and creating conditions for eventual resurgence in its Iraqi and Syrian heartland."

Additionally, U.S. estimates put the number of IS fighters and supporters in Syria and neighboring Iraq at between 14,000 and 18,000.

But U.S. and coalition officials worry the drawdown has made it more difficult for them and for partner forces to keep track of IS activity.

According to the new inspector general report, the U.S. drawdown came as partner forces needed more "training and equipping to build trust with local communities and to develop the human-based intelligence necessary to confront ISIS resurgent cells and insurgent capabilities in Syria."

Disinformation campaign

At the same time, officials warn the U.S.-backed SDF is falling victim to a damaging disinformation campaign.

"ISIS has been successful in portraying the SDF as the new occupying force in the area 'to exploit tension between the Kurdish-led SDF and local Arab residents,'" the inspector general report said, citing an assessment by U.S. Central Command.

FILE - A Kurdish policeman checks an Arab Syrian man at a checkpoint controlled by the U.S.-backed Syrian Democratic Forces on a highway in Hassakeh province, March 28, 2018.

Coalition military officials voiced similar concerns, noting Russia, Iran and the government of Syrian President Bashar al-Assad also "seek to weaken the SDF by leveraging Arab grievances against it, which could result in 'overall failure to maintain the mission [against ISIS] in Syria.'"

Adding to the concerns, coalition officials said efforts to increase the size of U.S. and coalition-backed forces are falling short.

CJTF-OIR said by the end of June, partner forces had a total of 100,000 troops — 10,000 less than needed.

U.S. officials have been trying to make up for the drawdown of U.S. forces in Syria by securing commitments from allied nations to send additional forces. But officials admit it has been a challenge.

"I would say that we are well on the way to getting formal commitments by a good number of countries, far more than where they are a year ago," U.S. Special Representative for Syria, Ambassador James Jeffrey, told a security forum last month.

"This will be a satisfactory outcome," he added.

But even if U.S. allies send more troops to Syria, U.S. officials admitted efforts to keep IS defeated in Iraq are also struggling.

U.S. military officials said IS still retains significant support, especially in northern and western Iraq, where it has been able to recruit and raise money.

The inspector general report cautioned that coalition officials also worry "Iraq lacks hold forces capable of maintaining security in areas cleared of ISIS."

IS "is able to operate as an insurgency in Iraq and Syria in part because the Iraqi Security Forces (ISF) and U.S.-backed Syrian Democratic Forces (SDF) remain unable to sustain long-term operations against ISIS militants," the report said.