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By Richard Engel, Caroline Radnofsky, Courtney Kube, Mosheh Gains and Saphora Smith

Four Americans were among those killed by an explosion in Syria on Wednesday, officials said.

Two American service members, a U.S. Defense Department civilian employee and a contractor supporting the department were killed while "conducting a routine patrol," according to a spokesperson for the U.S.-led coalition fighting the Islamic State, and a statement from U.S. Central Command.

Three other U.S. service members were also injured in the explosion in the Syrian city of Manbij, near the Turkish border, Central Command said in a statement. The names of those killed were not immediately released. Defense Department policy is to withhold names until 24 hours after notification of next of kin.

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 Manbij.

The scene of the suicide attack in the northern Syrian town of Manbij on Wednesday.Hawar News Agency / AFP - Getty Images

Coalition 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.

Two Manbij residents said the explosion took place in front of Kusr al-Umra, a downtown restaurant where U.S. troops routinely eat. Two other witnesses told NBC News that several American troops were inside the eatery at the time of blast, while others waited outside.

U.S. defense officials said the attack occurred while troops were on patrol. They had stopped at a restaurant when a suicide attacker blew himself up.

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.

American defense officials said that despite the claim of responsibility by the Islamic State, which is also known as ISIS, there has been little to no ISIS presence in Manbij in months, and the terror group sometimes falsely claims responsibility for attacks.

Wednesday’s killing of two U.S. service members was the single deadliest day for American forces in Syria.

Between 2014 and 2018, four service members died in the country, including two in combat, according to the Defense Department. Each of the four deaths occurred on different days.

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."

"Our deepest sympathies and love go out to the families of the brave American heroes who were killed today in Syria," a statement from the White House press secretary said. "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."

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.

CORRECTION (Jan. 18, 2019, 9:16 a.m. ET): An earlier version of this article misspelled the name of the Syrian city on the Turkish border where the U.S. service members were killed. It is Manbij, not Manjib.

Phil Helsel, Micah Grimes, Nick Bailey, Jason Cumming and Associated Press contributed.