The United States has no plan for Syria as it proceeds with President Donald Trumps order to pull American troops out of the country, a top official who quit in protest at the policy said on Sunday. Brett McGurk, who was Americas envoy to the US-led global coalition against ISIS, said "theres no plan for whats coming next" and this is increasing the risk to US forces. He spoke in an interview with CBSs "Face the Nation," after a suicide bomber on Wednesday killed four Americans and 15 others in the northern Syrian town of Manbij. It was the deadliest attack to hit US troops since they deployed to Syria in 2014 to assist local forces against ISIS. The bombing came after Trumps announcement last month that he was ordering a full withdrawal of the 2,000 US troops from Syria, shocking allies and leading to the resignations of Defense Secretary Jim Mattis as well as McGurk. McGurk, a longtime diplomat who was appointed special envoy by President Barack Obama in 2015, criticized Trumps decision to order the withdrawal of US troops, which he cast as a "total reversal" of years of US policy in the region. "On instructions from the White House, we were telling [U.S. partners on the ground that] wed stay in Syria until we completed the enduring defeat of ISIS, not just the physical territory. Wed stay in Syria until the Iranians left Syria, and also until there was a irreversible political progress in Geneva to end the ultimate civil war," McGurk said. "And it was the total reversal of our policy that made it — I just, I concluded I could not be effective in carrying out those new instructions." McGurk said a "sustainable" plan had been in place to defeat ISIS and begin serious negotiations with parties in Syria, including Russia. "The minute you announce to the world that youre leaving, a vacuum opens up and all the other powers in the region start making different calculations," he said. Still, givenTrumps announcement, he said the world "has to really accept some hard truths — I think, number one, we are leaving. The president has made that clear — we are leaving." Senior US officials have since given contradictory statements about US intentions, but the Pentagon said it had begun the withdrawal, although how long it would take remained uncertain. Trump announced the US withdrawal because, he said, ISIS had been defeated -- something McGurk and other experts dispute. McGurk has previously warned that the US pullout would shore up Syrias President Bashar al-Assad and lessen Americas leverage with Russia and Iran.