WASHINGTON — A suicide bombing claimed by the Islamic State killed at least 16 people, including two U.S. service members and two American civilians, in northern Syria on Wednesday, just a month after President Donald Trump declared that IS had been defeated and he was pulling out U.S. forces.

The attack in the strategic northeastern town of Manbij highlighted the threat posed by the Islamic State group despite Trump’s claims. It could also complicate what had already become a messy withdrawal plan, with the president’s senior advisers disagreeing with the decision and then offering an evolving timetable for the removal of the approximately 2,000 U.S. troops.

The attack, which also wounded three U.S. troops, was the deadliest assault on U.S. troops in Syria since American forces went into the country in 2015. The dead included a number of fighters with the Syrian Democratic Forces, who have fought alongside the Americans against the Islamic State, according to officials and the U.K.-based Syrian Observatory for Human Rights.

According to a U.S. official, one of the U.S. civilians killed was an intelligence specialist working for the Defense Intelligence Agency. The other was an interpreter, who was a contractor.

The attack prompted new complaints about the withdrawal and underscored Pentagon assertions that IS is still a threat and capable of deadly attacks.

In a Dec. 19 tweet announcing the withdrawal, Trump said, “We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency.” He said the troops would begin coming home “now.” That plan triggered immediate pushback from military leaders, including the resignation of the defense secretary.

Over the past month, however, Trump and others have appeared to adjust the timeline, and U.S. officials have suggested it will likely take several months to safely withdraw American forces from Syria.

Not long after the attack Wednesday, Vice President Mike Pence repeated claims of the Islamic State’s defeat. Speaking at the State Department, Pence said the “caliphate has crumbled” and the militant network “has been defeated.”

Others, immediately pointed to the attack as a reason to reverse or adjust the withdrawal plan.

Sen. Lindsey Graham, a Trump backer and prominent voice on foreign affairs on Capitol Hill, said during a committee hearing Wednesday he is concerned that Trump’s withdrawal announcement had emboldened the Islamic State and created dangerous uncertainty for American allies.