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.

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