The US-led coalition stated two of its members had been killed and five wounded by an improvised-explosive device (IED) around midnight on Thursday.
"Wounded personnel received immediate care and are being evacuated for further medical treatment," the coalition stated on Friday.
The two have been identified as an American and a Briton.
“The individual was embedded with US forces on a counter-Daesh operation when the incident occurred,” Britain’s defence ministry said of the death of its soldier, using an Arabic acronym for ISIS.
A Pentagon official has confirmed that the other soldier who died was an American, The Associated Press reported.
The coalition announced it started an investigation.
The Syrian Based Observatory for Human Rights reported four members of the Manbij Military Council were also injured.
The monitor did not lay blame on any party for Thursday's attack.
Earlier this week, the Observatory reported that Turkish intelligence agents were responsible for organizing an assassination attempt by Harakat al-Qiyam (Resurrection Movement) against Manbij Military Council Spokesperson Shervan Derwish.
The groupposted on a video on Twitter claiming the attack on Derwish.
The local military council promised to remain loyal to the US-led coalition and claimed Turkish-backed proxies were responsible for "chaos" in Manbij.
“As the Manbij Military Council, we are prepared to fight against anything that aims to interfere in our own affairs. We will defend Manbij, surely in partnership with the International Coalition, first and foremost the United States — that has taken the responsibility on its shoulder to defend Manbij,” a commander speaking on behalf of the MMC leadership said in a video statement on Thursday.
He said that some “cells” of the Turkey-backed Syrian proxies have tried to cause “chaos” among the city’s population, adding that the MMC is prepared to engage in fighting if attacked by Turkey and its Syrian militias.
The military and civil councils in northern Syria were formed after the US-led coalition backed the Syrian Democratic Forces to liberate "nearly 100 percent" of the territory ISIS once held in Syria.
Turkey rejected a French offer for mediation between it and the SDF.
“Who are you to talk about mediation between Turkey and a terror group?" said Turkish President Recep Tayyip Erdogan on Friday, according to state-run Anadolu Agency.
Harakat al-Qiyam was also responsible for an IED attack in November 2017 that killed Mohammad Abu Adel, the commander of the Manbij Military Council.
Omar Alloush, a co-chair of Raqqa Civil Council's Public Relations Committee and a member of the Syrian Democratic Forces (SDF) was killed in March. Harakat al-Qiyam claimed the assassination. It was condemned by the United States for being "violence on a civilian leader."
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