ISIS suicide attack Manbij Jan 16 2019

The scene of the suicide attack in the northern Syrian town of Manbij on Wednesday.
U.S. service members were among those killed by an explosion in Syria on Wednesday, a spokesperson for the coalition fighting the Islamic State group said.

The official said the Americans were killed while "conducting a routine patrol," but did not say how many.

Earlier, a senior Kurdish security official told NBC News that members of the U.S.-led coalition were caught up in a blast at a market in the northern city of Manbij. The official said that Americans were among the casualties but could not confirm the number of injured or dead.

Forces were on foot in the city when they were approached at around 1 p.m. local time (6 a.m. ET) by a man wearing civilian clothing with explosives hidden underneath, the Kurdish official added.

The blast happened in a market area of small alleys that is crowded with shops and street vendors.

Saif al-Din Tayyar, a journalist who said he was nearby when the explosion occurred, said he saw around 25 people either dead or wounded. NBC News could not immediately verify his report.

Witnesses told NBC News that military helicopters were sent to the scene to help deal with the casualties.

ISIS claimed responsibility for the attack through recognized social media accounts, claiming a suicide bomber wearing an explosive vest attacked coalition forces. ISIS did not immediately produce evidence to support the claim.

Manbij is near the border with Turkey.

White House press secretary Sarah Sanders told reporters that President Donald Trump had been "fully briefed and we will continue to monitor the ongoing situation in Syria."

Just hours after the attack, Vice President Mike Pence touted the administration's success defeating ISIS. "The caliphate has crumbled and ISIS has been defeated," Pence told diplomats at the Global Chiefs of Mission Conference in Washington D.C.

He added immediately afterwards that the U.S. would "stay in the region and we'll stay in the fight to ensure that ISIS does not rear its ugly head again," but did not specify where or what that would entail.

The attack comes weeks after Trump announced that he was withdrawing all U.S. troops from Syria immediately.

On Dec. 19, he tweeted that the U.S. had "defeated ISIS in Syria," adding that was the "only reason" to have troops on the ground there.

"We have won against ISIS. We have beaten them and we have beaten them badly," he said in a video posted on his Twitter account.

Administration officials later said the drawdown will happen more slowly.

National security adviser John Bolton said on Jan. 6 that American troops will not leave northeastern Syria until ISIS is defeated and American-allied Kurdish fighters are protected.

Turkey has threatened to attack the Kurds, who have been America's partners on the ground in the war against ISIS in Syria. Turkey views the Syrian Kurds, or YPG Kurdish militia, as a terrorist organization and part of the outlawed Kurdistan Workers Party.

However, a U.S. military official on Friday confirmed that the U.S.-led military coalition had begun the process of withdrawing troops from Syria.