US service members were among those killed in a suicide attack Wednesday in the flashpoint Syrian city of Manbij, the US-led coalition said without specifying how many.
"US service members were killed during an explosion while conducting a routine patrol in Syria today," a coalition spokesman said in a statement posted on social media.
"We are still gathering information and will share additional details at a later time."
According to the Syrian Observatory for Human Rights, at least two US members of the coalition were killed in the attack targeting their patrol.
The Britain-based monitoring group relies on a network of sources inside Syria for its information.
The attack, the first of its kind against the coalition in Manbij in 10 months, was claimed by the Islamic State jihadist group.
According to casualty statistics from the Pentagon, Wednesday's blast will go down as the worst loss of US life in Syria since the coalition was created in 2014.
The US Department of Defense previously only reported two US nationals being killed in combat in Syria, in two separate incidents.
US President Donald Trump announced a month ago that he was ordering a full troop pullout from Syria, saying the war against IS was won.