Donald Trump’s decision to withdraw troops from Syria has left US allies ill-equipped to deal with a resurgent Isis, according to a new Pentagon report.

Isis has been steadily rebuilding its capabilities since the defeat of its caliphate in March this year. In the time since, it has established “resurgent cells” in the country and carried out assassinations, suicide attacks, abductions and arson of crops, the report said.

But the withdrawal of some 2,000 US soldiers 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 drawdown has left the US-backed Syrian Democratic Forces, Washington’s partner on the ground in the fight against Isis, with “limited capacity to hold liberated areas”.

The assessment, authored by the Defence Department’s inspector general and based on interviews with top US military commanders, is a direct contradiction of Mr Trump’s repeated assertions that Isis has been defeated, and suggests that the president’s strategy may in fact be giving the group space to rebuild.

Syria at war: Fleeing the caliphate Show all 14 1 /14 Syria at war: Fleeing the caliphate Syria at war: Fleeing the caliphate Trucks full of women and children arrive from the last Isis-held areas in Deir ez-Zor, Syria Richard Hall/The Independent Syria at war: Fleeing the caliphate Zikia Ibrahim, 28, with her two-year-old son and 8-month-old daughter, after fleeing the Isis caliphate Richard Hall/The Independent Syria at war: Fleeing the caliphate Trucks full of women and children arrive from the last Isis-held areas in Deir ez-Zor, Syria. Richard Hall Richard Hall/The Independent Syria at war: Fleeing the caliphate Men who fled the last Isis-held area of Syria line up to be questioned by American and Kurdish intelligence officials Richard Hall/The Independent Syria at war: Fleeing the caliphate Trucks full of women and children arrive from the last Isis-held areas in Deir ez-Zor, Syria. Richard Hall Richard Hall/The Independent Syria at war: Fleeing the caliphate A young girl pulls her belongings after arriving Richard Hall/The Independent Syria at war: Fleeing the caliphate An SDF fighter hands out bread to women and children after they arrive Richard Hall/The Independent Syria at war: Fleeing the caliphate Sita Ghazzar, 70, after fleeing from the last Isis-held territory in Syria Richard Hall/The Independent Syria at war: Fleeing the caliphate A family from Russia who recently fled the last Isis-held area of Syria Richard Hall/The Independent Syria at war: Fleeing the caliphate Trucks full of women and children arrive from the last Isis-held areas in Deir ez-Zor, Syria. Richard Hall Richard Hall/The Independent Syria at war: Fleeing the caliphate Trucks full of women and children arrive from the last Isis-held areas in Deir ez-Zor, Syria. Richard Hall Richard Hall/The Independent Syria at war: Fleeing the caliphate Trucks full of women and children arrive from the last Isis-held areas in Deir ez-Zor, Syria. Richard Hall Richard Hall/The Independent Syria at war: Fleeing the caliphate Trucks full of women and children arrive from the last Isis-held areas in Deir ez-Zor, Syria. Richard Hall Richard Hall/The Independent Syria at war: Fleeing the caliphate Richard Hall Richard Hall/The Independent

Mr Trump announced the decision to withdraw US troops on Twitter in December, writing: “We have defeated Isis in Syria, my only reason for being there during the Trump presidency.”

The move shocked officials within Mr Trump’s own administration, who argued that it would leave America’s Kurdish allies on the ground vulnerable, and that Isis was not yet truly finished. The move prompted the resignation of the defence secretary Jim Mattis and the head of the anti-Isis coalition, Brett McGurk.

Mr Trump later walked back on his plan for a full withdrawal, instead opting to leave a small residual force of a few hundred US troops in the country.

Far from being defeated, the Pentagon report details how Syrian and Iraqi forces are struggling to contain a renewed Isis insurgency.

“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 said.

The report noted that Isis retains between 14,000 and 18,000 “members” in Iraq and Syria, including up to 3,000 foreigners. But the group is still actively trying to recruit new fighters in both countries.

Isis is using family and tribal connections to recruit in northern and western provinces of Iraq, it said, while in Syria, the Pentagon is growing increasingly concerned over the situation in al-Hol camp, where thousands of Isis family members are being held by the SDF.

The US-led coalition set up to defeat Isis told the report’s authors that the camp is protected by “minimal security due to the US drawdown”, and that Isis is likely “exploiting the lack of security to enlist new members and re-engage members who have left the battlefield”.

The deputy commander of the coalition recently told The Independent that the camp represented “the biggest long-term strategic risk” in the fight against the jihadi group outside of ongoing military operations.

The Pentagon report follows a similarly grim outlook assessment from the United Nations earlier this week. UN secretary-general Antonio Guterres said in a report circulated on Monday that Isis has been left with as much as $300m (£247m) following the loss of its caliphate, “with none of the financial demands of controlling territory and population”.