WASHINGTON (AP) — A suicide bombing claimed by Islamic State militants 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 group, 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 group's defeat. Speaking at the State Department, Pence said the "caliphate has crumbled" and the militant network "has been defeated." Later in the day he released a statement condemning the attack but affirming the withdrawal plan.

"As we begin to bring our troops home, the American people can be assured, for the sake of our soldiers, their families, and our nation, we will never allow the remnants of ISIS to re-establish their evil and murderous caliphate - not now, not ever," he said.

Others, however, 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 Islamic State militants and created dangerous uncertainty for American allies.

"I know people are frustrated, but we're never going to be safe here unless we are willing to help people over there who will stand up against this radical ideology," he said.

Rep. Elissa Slotkin, D-Mich., said the attack demonstrates the lethal capability of IS and "the fact that it happened in Manbij, probably the single most complicated area of Syria, demonstrates that the president clearly doesn't understand the complexity of the problem."

Manbij is the main town on the westernmost edge of Syrian territory held by the U.S.-backed Syrian Kurds, running along the border with Turkey. Mixed Kurdish-Arab Syrian forces liberated Manbij from IS in 2016 with help from the U.S.-led coalition.

But Kurdish control of the town infuriated Turkey, which views the main U.S. Kurdish ally, the YPG militia, as "terrorists" linked to Kurdish insurgents on its own soil.

The town has been at the center of tensions in northern Syria, with the militaries of two NATO members, the U.S. and Turkey, on opposing sides. The two sides began joint patrols around Manbij in November as part of an agreement aimed at easing tensions.

Slotkin, a former senior Pentagon adviser on Syria and other international issues, said it's time for Trump to amend or change his withdrawal order to "something more consistent with the threat" in Syria.

Others suggested the attack could trigger change.

"Certainly the Islamic State follows the news closely, and observing the recent controversy over a potential withdrawal would incentivize them to try for a spectacular attack to sway both public and presidential opinion," said Jim Stravidis, a retired Navy admiral who served as top NATO commander.

Trump, meanwhile, reinforced his withdrawal decision during a meeting with about a half-dozen GOP senators late Wednesday at the White House.

Sen. Rand Paul of Kentucky, who was at the meeting, told reporters on a conference call that the president remained "steadfast" in his decision not to stay in Syria - or Afghanistan - "forever." But the senator did not disclose the latest thinking on withdrawal timeline.

Paul, who has been one of the few voices in the GOP encouraging the president's noninterventionist streak, said Trump told the group, "We're not going to continue the way we've done it."

Video of Wednesday's attack released by local activists and news agencies showed a restaurant that suffered extensive damage and a street covered with debris and blood. Several cars were also damaged. Another video showed a helicopter flying over the area.

A security camera showed a busy street, and then a ball of fire engulfing people and others running for cover as the blast went off.

The names of the American victims are being withheld until their families can be notified.

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Associated Press writer Lisa Mascaro contributed to this report.