The Pentagon identified a US service agent on Saturday, who died Friday after he exploded near his patrol in Syria the day before.
Master Sgt. Jonathan Dunbar, 36, of Austin, Texas, is the second American to have died in Syria since the US supports local forces in a conflict sworn by President Trump.
The attack on Thursday took place during an operation against the Islamist state, which also left a British service member. Five more were injured.
The deaths occurred about two hours after Trump's speech in Ohio had promised to "retreat" around 2,000 US troops in Syria and "care for other people".
The incident that took place in the city of Manbij in northern Syria is being investigated, said the Ministry of Defense.
Dunbar became the headquarters of the Special Operations Command of the US Army in Ft. Bragg, N. C. He joined the army in 2005 and was stationed three times in Afghanistan and Iraq. He received three bronze stars.
The term "headquarters" for soldiers assigned to the command was linked to the secret defense unit of the Delta Force .
Also killed was Sgt. Matt Tonroe, a British soldier who served in the 3rd Battalion of the Elite Parachute Regiment. Tonroe had been used several times for operations in Afghanistan and the Middle East.
Trump urges the removal of US troops from Syria and says it makes little sense for the United States to have so many forces in the country when they have almost won the war against the Islamic State. His remarks were not planned, and it was not clear what prompted him to mention Syria in a speech about the infrastructure. A government official said it could take a year or more before such a move happens.
The presidential aides persuaded him to stay for the time being to prevent the Islamic State from reappearing and to lay the foundations for a possible peace agreement that could benefit the United States.