This frame grab from video provided by Hawar News, ANHA, the news agency for the semi-autonomous Kurdish areas in Syria, shows a damaged restaurant where an explosion occurred, in Manbij, Syria, Wednesday, Jan. 16, 2019.

Wednesday's attack on U.S. forces in Syria has stoked fresh criticism over President Donald Trump's claim that the so-called Islamic State has been defeated renewed debate over his decision to withdraw all troops from the war-torn country.

Around 1 p.m. local time, a suicide bomber blew himself up in a popular area of downtown Manbij, a northern Syrian city that's been controlled by U.S.-supported Kurdish militias since it was wrested from ISIS in 2016.

Four Americans were killed — two service members, a civilian Pentagon official and a U.S. contractor — and three more injured, U.S. Central Command confirmed in a statement, reportedly marking the largest single loss of American life since the counter-ISIS campaign began. Nineteen people are believed to have died in total, including civilians and local coalition partners, according to monitoring group the Syrian Observatory for Human Rights.

ISIS quickly claimed responsibility. While the group has not so far offered physical evidence to support the claim, critics have been quick to link the attack to President Donald Trump's decision last month to withdraw all U.S. troops from Syria.

"Trump's order was reckless and driven far more by domestic political concerns than it was by facts on the ground," Charles Lister, a senior fellow at the Middle East Institute, said on Twitter Wednesday.

Trump defended the troop pullout plans on the premise that ISIS had been defeated. The decision triggered rebukes from numerous lawmakers and security experts, who warned of the extremist group's resurgence and lamented what was seen as an abandonment of local partners.

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"Sometimes reality catches up quickly with wishful thinking and political spin," Michael Rubin, a former Pentagon official and resident scholar at the American Enterprise Institute in Washington, D.C., told CNBC on Wednesday. "Historians will likely file Trump's tweets announcing the ISIS defeat and U.S. Syria pullout alongside Bush's 2003 'Mission Accomplished' speech and Obama's 2011 withdrawal from Iraq."