AVA- Reuters quoted an unnamed US official as saying that three other American troops were also wounded in the Wednesday afternoon blast.
"US service members were killed during an explosion while conducting a routine patrol in Syria today," the military headquarters responsible for operations in Iraq and Syria had earlier said in a statement. "We are still gathering information and will share additional details at a later time."
The so-called Syrian Observatory for Human Rights (SOHR) identified nine of the fatalities as civilians, adding that the rest were members of the US-backed Syrian Democratic Forces (SDF), which has Kurdish People's Protection Units (YPG).
The Britain-based monitor added that the blast targeted the al-Ummara restaurant in the center of Manbij on Wednesday afternoon.
The Observatory noted that the death toll is expected to rise as some of the wounded victims are in critical condition.