The explosion in Afrin wounded more than a dozen others, hitting an industrial area during the afternoon rush hour on the first working day of the week.
Explosions in northern regions controlled by opposition fighters backed by neighboring Turkey have left scores of people dead or wounded over the past months.
Efrinnews 24, an activist collective that covers events in Afrin, said Saturday's blast killed four Turkey-backed opposition fighters and wounded others.
Turkey and allied Syrian fighters took control of Afrin in 2018 in a military operation that expelled local U.S.-backed Kurdish fighters and displaced tens of thousands of Kurdish residents.
Ankara considers the Kurdish fighters who were in control of Afrin a national security threat. Since then, there have been a series of attacks on Turkish targets in the area.