A suicide bomber triggered a fiery explosion Wednesday that killed four U.S. citizens at a market in northern Syria, less than a month after President Donald Trump declared victory over Islamic State militants in the region.

Two of the dead were U.S. soldiers, and two others were U.S. civilians, military officials said. The Islamic State, also known as ISIS, claimed responsibility for the attack in the town of Manbij.

The attack complicates a messy plan for U.S. withdrawal, a decision Trump's senior advisers disagreed with before offering an evolving timetable for the removal of the approximately 2,000 U.S. troops. The bombing also underscores Pentagon assertions that the Islamic State is still a threat capable of deadly attacks.

"U.S. service members were killed during an explosion while conducting a routine patrol in Syria today," the military's Operation Inherent Resolve said in a statement.

A total of 16 people were killed in the midday explosion, including fighters with Syrian Democratic Forces. Those troops have fought alongside Americans in skirmishes with the Islamic State.

U.S. Central Command said one of the civilians was employed by the Defense Department and the other was a contractor. The names of the American victims were withheld until their families could be notified.

Vice President Mike Pence repeated Trump's claims about the Islamic State on Wednesday, saying the "caliphate has crumbled" and the militant network "has been defeated."

The Syrian Observatory for Human Rights, a Britain-based group that monitors the conflict in Syria, said the cause of the explosion was a suicide bomb outside a restaurant. The group said the attack killed at least 16 people.

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The White House said Trump was fully briefed on the situation.

"Our deepest sympathies and love go out to the families of the brave American heroes who were killed today in Syria," the White House said in a statement. "We also pray for the soldiers who were wounded in the attack.

"Our service members and their families have all sacrificed so much for our country."

Manjib is controlled by the U.S.-backed Kurdish People's Defense Units. The explosion took place in a market wedged along a street thick with cars. Video that Hawar identified as from the scene shows people gathered on a crowded sidewalk when the fiery blast occurred.

A senior Kurdish security official told NBC News the U.S. forces were on foot in the city when they were approached by a man wearing civilian clothing that covered the hidden explosives.

Since 2016, four U.S. troops had been killed in Syria before the latest attack, according to Pentagon records. Those troops took part in Operation Inherent Resolve, the U.S.-led effort begun in 2014 to combat Islamic State terrorists in Iraq and Syria.

Last month, Trump announced that the 2,000 U.S. troops in Syria would be withdrawn. “We have won against ISIS.” Trump said. “Now it’s time for our troops to come back home.”

Trump’s decision to pull out of Syria was one of the reasons Defense Secretary Jim Mattis cited for stepping down last month. Mattis urged Trump to maintain the U.S. presence to aid Kurdish allies and deter the Syrian and Iranian regimes.

After a series of confusing signals from the Pentagon and the White House, officials announced last week that equipment had been removed from U.S. facilities in Syria and that all troops would withdraw within months.

Sen. Lindsey Graham, R-S.C., again called on the president to rethink his plans.

“I hope the president would look long and hard about where he’s headed in Syria,” Graham, one of Trump’s closest allies in the Senate, said Wednesday, interrupting a Senate Judiciary Committee confirmation hearing on William Barr’s nomination as attorney general. “I don’t know how we can be safe unless we give (U.S. allies in Syria) the space to be safe.”

Contributing: Kevin Johnson; The Associated Press