US-led coalition Operation Inherent Resolve did not mention how many service members were killed. Prior to Wednesday's attack, only two US service members had been killed in action in Syria since the start of the campaign in 2014.
"US 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," according to a tweet from the spokesperson for the Operation Inherent Resolve.
ISIS has claimed responsibility for the deadly explosion.
According to the Syrian Observatory for Human Rights monitoring group, at least 15 people, including a US serviceman, were killed in the rare attack in Manbij. It said the cause of the explosion was a suicide bomb outside a restaurant.
The explosion took place in a market wedged along a crowded street thick with cars. Video that Hawar identifies as from the scene shows people gathered on a crowded sidewalk when the fiery blast occurs.
The attack comes less than a month after Trump announced that US troops would withdraw from Syria.
Trump at that time declared in a video released on Twitter: "We have won against ISIS. We've beaten them and we've beaten them badly. We've taken back the land and now it's time for our troops to come back home."
The US has about 2,000 troops in Syria, with no specific date for their withdrawal. Last week, the US began withdrawing some military ground equipment from Syria, according to an administration official with direct knowledge of the operation.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)