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(CNN) —  

The former US envoy for the anti-ISIS coalition said Monday that “ISIS is not defeated” nearly a month after President Donald Trump announced he was withdrawing troops from Syria on the premise that the US had defeated the terrorist group.

Brett McGurk, who resigned last month because of Trump’s decision, made the comment in an interview with CNN’s Christiane Amanpour just hours after an attack targeting US forces in the country occurred. It also comes just days after a deadly attack by ISIS in northern Syria killed four Americans and at least 10 other people in the city of Manbij. No Americans were killed in Monday’s attack, and the US did not immediately blame a particular terrorist group.

“In early December, Secretary Mattis and I met with all the military contributors of our coalition, including many countries that had been attacked from ISIS out of Syria and the unanimous view is that ISIS is not defeated, this mission is not over,” McGurk told Amanpour. “I do not think there would be a single expert that would walk in the Oval Office and tell the President that this is over.”

In an op-ed penned in The Washington Post last week, McGurk argued that 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,” McGurk wrote.

McGurk also told Amanpour that the withdrawal meant abandoning leverage over Russia in the region.

“We were getting to point where with the defeat of the physical caliphate, we would be able to sit with the Russians and have a very serious conversation about the future of Syria,” he said. “Announcing to the world that we were just leaving, basically all of that leverage completely evaporates. It’s going to be a very different conversation now.”

McGurk – who told Amanpour he never met Trump – also criticized the way the President made the decision. The US President told Turkish President Recep Tayyip Erdogan the US was “done” with Syria ahead of his decision, a senior administration official who received a detailed readout of the phone call between both presidents has told CNN.

“Well again, to help design a campaign plan that was succeeding, and that was reaching a really critical phase and we were talking about the longer term transition and to have it all upended in a phone call with a foreign leader without any serious consultation with the national security team, with the secretary of defense or others, that’s just not the way to run foreign policy effectively,” McGurk said.

As a result of the decision, which also caused former Defense Secretary James Mattis to resign from his post, citing irreconcilable policy differences, McGurk told Amanpour that his own authority on the issue was shattered.

“I basically concluded my effectiveness, my credibility to carry out those new instructions, it would really – I was unable to do it,” he said.

CNN’s Mick Krever and Veronica Stracqualursi contributed to this report.