Washington CNN  — 

The former US envoy for the anti-ISIS coalition, who resigned last month because of the President’s abrupt decision to pull US forces out of Syria, has penned an op-ed arguing that Donald Trump has given the terrorist group “new life.”

“The irony is that defeating the Islamic State is what the president said from the beginning was his goal. In 2016, he vowed to ‘knock the hell out of ISIS.’ His recent choices, unfortunately, are already giving the Islamic State — and other American adversaries — new life,” Brett McGurk, the former special presidential envoy for the Global Coalition to Counter ISIS, wrote in an Washington Post op-ed published Friday.

McGurk’s comments come two days after four Americans were killed in an attack in Syria. CNN reported Thursday that the US initial assessment is that ISIS is responsible for the attack.

The explosion in the northern city of Manbij Wednesday killed the four Americans and at least 10 other people. Eight civilians and two fighters from the US-backed Syrian Democratic Forces were killed in the blast, a senior commander from the Manbij military council told CNN.

McGurk also argued that ISIS will “fill the void opened” by US troops’ departure and become a threat again to Europe and ultimately the US.

He warned that Turkey is not a reliable partner because groups it backs are “marbled with extremists.” He argued that if the US-backed Syrian Democratic Forces don’t find a new benefactor, it risks “fracturing and opening a vacuum into which the Islamic State can resurge.”

McGurk, who was appointed to the envoy role in 2015, wrote that key gains in the fight against ISIS have been due to the “small and highly effective American military presence in Syria.”

He opined that the US should narrow its objectives in Syria to: “mitigating the risk of an Islamic State resurgence and preventing Iran from fortifying a military presence that threatens Israel.”

“These narrow objectives would be unsatisfying for those with greater hopes for Syria,” McGurk wrote. “But those hopes are dead.”

Acting chief Pentagon spokesman Charlie Summers would not comment on McGurk’s op-ed.

“He’s a private citizen and he can say whatever he likes,” Summers told reporters Friday, adding that “we remain committed to the defeat of terrorism.”

Going against the advice of his military and national security advisers, Trump announced on December 19 the “full” and “rapid” withdrawal of US military from Syria, declaring that the US has defeated ISIS. McGurk, soon after Defense Secretary James Mattis resigned in part over the Syria decision, notified Secretary of State Mike Pompeo that he too would be leaving.

“The president’s decision to leave Syria was made without deliberation, consultation with allies or Congress, assessment of risk, or appreciation of facts,” McGurk wrote in The Post op-ed.

He said that only Russia and Iran have praised Trump’s decision and that “whatever leverage we may have had with these two adversaries in Syria diminished once Trump said we would leave.”

“These trends will worsen if the president does not reverse course: Our partners will stop listening and make decisions that run contrary to our interests,” he wrote. “Our adversaries will play for time, knowing the United States is on its way out.”

The US has about 2,000 troops in Syria, with no specific date for their withdrawal. Last week, the US began withdrawing some military ground equipment from Syria, according to an administration official with direct knowledge of the operation.

On Friday, Summers refused to answer reporters’ questions whether or not ISIS had been defeated in Syria, only adding that “there are pockets (of ISIS), certainly.”

He later issued a statement saying that Wednesday’s ISIS-claimed suicide bombing in Syria that killed four Americans demonstrates “ISIS remains a threat.”

CNN’s Ryan Browne contributed to this report.