Updated at 5:42 p.m. ET

Four Americans were killed in an explosion while conducting a routine patrol in northern Syria, according to the Pentagon. The Islamic State has claimed responsibility.

Two U.S. service members, one civilian employee of the Defense Intelligence Agency and one contractor working as an interpreter died in the attack in Manbij. Three service members were injured.

A statement from U.S. Central Command, which oversees operations in the Middle East, says that initial reports indicate an explosion caused the casualties, but that the incident is under investigation. U.S. forces with the international coalition regularly patrol in and around the town.

"President Trump and I condemn the terrorist attack in Syria that claimed American lives and our hearts are with the loved ones of the fallen," Vice President Pence said in a statement.

A local news site reported that a huge explosion erupted in the city center near a girls' school and a restaurant. The site reported that both civilians and troops were killed and wounded. Local groups say at least 16 people were killed in total, according to The Associated Press.

ANHA, a news agency in the Kurdish areas in Syria, showed the restaurant's windows blown out, with the twisted metal frame of an awning hanging off the building.

The town of Manbij, located close to the Turkish border in northern Syria, was retaken from ISIS in 2016. U.S. troops have been working in the city with the local military council, as well as patrolling outside the city with Turkish troops, NPR's Tom Bowman reports.

"It's a vibrant, bustling city," says Bowman, who visited in early 2018. "[It] has a huge market selling all sorts of goods and produce."

"I was there ... with the U.S. military," Bowman says, "We walked around without body armor. It was remarkably calm. ... You would never get a sense that there was a shot fired in anger there."

But some residents and members of the U.S. military have been concerned about ISIS fighters "slipping back into the city" for more than a year, Bowman reports.

The U.K.-based Syrian Observatory for Human Rights group says that a suicide bomber probably carried out the attack.

Hassan Hassan, an expert on the Islamic State, says the group has identified the bomber as Abu Yasin al-Shami.

The explosion comes shortly after President Trump announced in December that the U.S. would withdraw forces from Syria. The announcement put him at odds with some of his advisers and worried U.S. allies. Defense Secretary Jim Mattis and the U.S. envoy to the coalition fighting the Islamic State, Brett McGurk, resigned in response to the decision.

Pence hinted Wednesday that the fight against ISIS was nearly finished.

"Thanks to the courage of our Armed Forces, we have crushed the ISIS caliphate and devastated its capabilities," Pence said. "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 reestablish their evil and murderous caliphate – not now, not ever."

Lawmakers on both sides of the aisle took a different view.

Sen. Lindsey Graham, R-S.C., said he believed he had been in the restaurant that was bombed while he visited with Kurds, Arabs and others in Manbij. He urged the president to "look long and hard of where he's headed in Syria."

Graham said: "My concern about the statements made by President Trump is that you have set in motion enthusiasm by the enemy we're fighting. ... Every American wants our troops to come home, but I think all of us want to make sure that when they do come home, we're safe."

President Trump has said U.S. allies could complete the job of dismantling the extremist group. In December, Trump said, "We'll be coming out of Syria, like, very soon. Let the other people take care of it now."

The Pentagon says the U.S. has begun withdrawing troops, NPR reports.

About 2,200 American troops serve in Syria, working with Arab and Kurdish rebels to defeat the Islamic State, reports NPR's Tom Bowman.

Now that the U.S. is leaving — and no one is offering a timetable, by the way — the concern is that the Arab and Kurdish forces won't be able to finish the job on their own. They just don't have the strength. So ISIS could expand. That's the main concern. And - or there could be some sort of power struggle among the rebels, maybe ethnic cleansing, a possible bloodbath, one official told me.

Sen. Chris Coons, D-Del., said Wednesday's attack underscores the danger in Trump's policy.

"The tragic death of three American service members, which has been recently reported, is a reminder both of how lethal ISIS is and how risky an abrupt pullout can be, that it may well encourage ISIS to be more aggressive as our forces begin to depart," he told reporters.

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