A car bomb Thursday killed oil workers and fighters from a US-backed force battling the Islamic State group in east Syria, the Kurdish-led alliance said, with a monitor putting the death toll at 20.
A spokesman for the Syrian Democratic Forces (SDF) said the attack killed mostly oil workers, as well as fighters in the village of Shheel close to an oil field acting as a base for the anti-IS force.
Jihadist "cells are trying to stop our progress," Adnan Afrin told AFP. The Britain-based Syrian Observatory for Human Rights said 14 oil workers and six conscripts died after "the car bomb was detonated remotely".
The SDF fighters were escorting the workers in vehicles from the Omar oil field, the monitor said.
The SDF are fighting to expel the jihadists from the last shred of their "caliphate" in the village of Baghouz, some 100 km (60 miles) south along the Euphrates river.
The SDF took control of the Omar oil field in October 2017, and have turned it into their main base in the area.
IS seized large swathes of Syria and neighbouring Iraq in 2014, declaring a "caliphate" there, but have since lost all but the Baghouz holdout to various offensives.