American troops were killed in an explosion in northern Syria, the spokesperson for the U.S.-led coalition against the Islamic State says. The ISIS extremist group has claimed responsibility.

"U.S. service members were killed during an explosion while conducting a routine patrol in Syria today. We are still gathering information and will share additional details at a later time," Col. Sean Ryan, a spokesman for Operation Inherent Resolve, said in a statement.

Reuters, citing an unnamed U.S. official, reports that four U.S. troops were killed in the attack in Manbij. U.S. forces with the international coalition routinely patrol in and around the town.

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

The Islamic State claimed responsibility for the attack. Hassan Hassan, an expert on the Islamic State, says the grouphas identified the bomber as Abu Yasin al-Shami.

White House press secretary Sarah Sanders said in a statement: "The president has been fully briefed and we will continue to monitor the ongoing situation in Syria."

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.

At the time, the president claimed the fight against the Islamic State was nearly finished and that U.S. allies could complete the job.

"And by the way, we're knocking the hell out of ISIS," 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.

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