An Islamic State-affiliated web site, Amaq, said an attacker with an explosive vest had struck a foreign military patrol in a suicide attack.
Reuters could not independently verify a report by the UK-based Syrian Observatory for Human Rights that 14 people had been killed in the attack including two U.S. soldiers. The coalition could not be immediately reached for comment.
Last month, U.S. President Donald Trump made a surprise announcement that he would withdraw all 2,000 U.S. troops from Syria after concluding Islamic State had been defeated there. The announcement rattled allies in the region and top U.S. officials, including Secretary of Defence Jim Mattis who quit.
Two witnesses described Wednesday's blast to Reuters. The U.S.-backed, Kurdish-led Manbij Military Council militia that controls the town said there was preliminary information that people had been injured in Wednesday's attack.
Manbij has been held by U.S.-backed fighters allied to the Kurdish YPG militia since they took it from Islamic State in 2016. It is located near areas held by Russian-backed Syrian government forces and by anti-Assad fighters backed by Turkey.
One of the witnesses said there was a "heavy" presence of military aircraft over Manbij following the blast, which took place near a vegetable market. (Reporting By Angus McDowall and Ellen Francis; Editing by Peter Graff)