A suicide bomber is reported to have targeted US-led coalition forces in the Kurdish-held Syrian town of Manbij.
There was no confirmation from the coalition, but a Kurdish news agency says two American troops, one Kurdish fighter and nine civilians were killed.
Video posted online showed a helicopter taking off from the town apparently evacuating those wounded by the blast.
The jihadist group Islamic State (IS) said one of its members had detonated an explosive vest beside a patrol.
US soldiers are deployed in Manbij to support an alliance of Kurdish and Arab militias that has driven IS out of almost all of eastern Syria.
Last month, President Donald Trump announced that the US would begin pulling out all its 2,000 troops from Syria because IS had been “defeated”.
The move shocked allies and drew criticism from US lawmakers. Defence Secretary James Mattis and several other senior officials subsequently resigned.
Opponents of the withdrawal warned that Manbij and other towns near the border with Turkey might come under attack by the Turkish military, which has threatened to invade to clear them of the Kurdish People’s Protection Units (YPG) militia.
The Turkish government considers the YPG an extension of the banned Kurdistan Workers’ Party (PKK), which has fought for Kurdish autonomy in Turkey for three decades. However, it denies any direct organisational links to the group.