At least four Americans were killed in a suicide bombing claimed by ISIS in north Syria Wednesday — less than a month after President Donald Trump announced the U.S. had defeated the terror group in the country.

The explosion, targeting a busy market area in the Kurdish-held northern city of Manbij, killed 16 people when it struck at 1 p.m. local time (6 a.m. ET), according to U.K.-based monitoring group the Syrian Observatory for Human Rights.

Central Command (Centcom), which oversees U.S. military operations in the Middle East, confirmed that four Americans were among those killed in the attack.

"Two US service members, one Department of Defense (DoD) civilian and one contractor supporting DoD were killed and three service members were injured while conducting a local engagement in Manbij, Syria, Jan. 16, 2019,” the statement said.

Centcom added that the incident was under investigation, but offered little else in the way of details: “Initial reports indicate an explosion caused the casualties, and the incident is under investigation."

ISIS, via its propaganda arm Amaq, claimed responsibility within an hour of the blast, saying the attacker had used an explosive vest to target coalition forces who regularly patrol the city. It claimed nine U.S. soldiers had been killed or wounded in the attack.

Before Wednesday, only two U.S. servicemembers had been killed in action in Syria since the campaign began, in 2014. When the dust settles, the suicide bombing in Manbij could mark the deadliest attack against U.S. forces in the conflict.

The blast comes less than a month after Trump’s shock announcement that he was pulling U.S. troops out of Syria, claiming “we have won against ISIS. We've beaten them and we've beaten them badly.” The move to withdraw the 2,000-odd American troops in the country caused widespread consternation in the Pentagon, and criticism that Washington would be abandoning the Kurds, its key partner in the fight, to aggression from neighboring Turkey as well as the Syrian regime and its backers.

The decision drove Defense Secretary Jim Mattis and the U.S. envoy for the global coalition against ISIS, Brett McGurk, to resign in protest.

Confusion has continued since then. Despite Trump’s abrupt announcement, no troops have left yet, and the roadmap for withdrawal remains murky. Last week, White House national security adviser John Bolton laid out conditions for the withdrawal that could see a lengthy American presence in the conflict, only to be undercut by officials in the Pentagon who said the withdrawal had already begun.

“Nothing has changed. We don’t take orders from Bolton,” a defense official told the Wall Street Journal, adding that the Pentagon planned to follow through on the president’s directive.

Wednesday’s attack highlights that Trump’s decision to pull out was “extremely premature,” said Charles Lister, Director of Countering Terrorism and Extremism at the Middle East Institute, who described the guerrilla nature of the attack as “ISIS 101.”

“Today’s attack shows to all concerned not only how dangerous that decision was, but that the reality on the ground is not at all what the president told his people and the battle against ISIS is far from over,” he said.

“Not only are the U.S. and their local partners still engaged in open warfare against ISIS in eastern Syria, but there have also been clear signs for many months that ISIS maintains the ability to conduct a low-level guerrilla-style insurgency in Syria, as typified by today’s attack.”

White House spokeswoman Sarah Huckabee Sanders said shortly after the blast that Trump had been fully briefed. “We will continue to monitor the ongoing situation in Syria,” she told reporters. The administration didn't seem to be wavering on its controversial claims, however. Just hours after the deadly attack, Vice President Mike Pence repeated his boss’s position that ISIS had been beaten. “The caliphate has crumbled and ISIS has been defeated,” he told diplomats at the Global Chiefs of Mission Conference in Washington, D.C.

Cover: In this Wednesday, April 4, 2018 file photo, a U.S. soldier, left, sits on an armored vehicle behind a sand barrier at a newly installed position near the front line between the U.S-backed Syrian Manbij Military Council and the Turkish-backed fighters, in Manbij, north Syria. The U.S. military said Friday it has started pulling equipment, but not troops, out of Syria as a first step in meeting President Donald Trump's demand for a complete military withdrawal. (AP Photo/Hussein Malla, File)