According to the official Sana news agency, at least eleven Syrian government soldiers and two civilians were killed in an ambush near the remote eastern district of Raqqa on Monday morning.
The attack targeted a bus around 6:30 a.m. local time on a road between Raqqa, a former Islamic State (IS) stronghold now under Kurdish administration, and Homs, a city under government control. Three more persons were injured in the incident.
The Syrian Observatory for Human Rights (SOHR), headquartered in the United Kingdom, confirmed at least 15 fatalities and warned that the death toll could grow.
Sana stated that a civilian bus was attacked while on the road. In contrast, SOHR reported that it was a military vehicle and that the incident occurred as Syrian government forces conducted an operation against IS terrorists.
No immediate claim of responsibility for the incident was made.
Northwest Syria is the final significant rebel bastion in the country's 11-year civil conflict against the Assad regime. The Turkish military occupies the territory held by rebels, which supports many rebel factions.
In May, a series of missile attacks on buses transporting the Syrian military resulted in the deaths of ten soldiers and the injury of nine more in the country's north.
Several years have passed since the main frontlines of the conflict, which erupted in 2011 from rallies against President Bashar al-Assad, froze.