The Syrian Democratic Forces (SDF) said they gained control of al-Omar oil field, one of Syria’s largest, on the eastern bank of the Euphrates river.
With air strikes and special forces from the U.S.-led coalition, the SDF has been battling Islamic State in oil-rich Deir al-Zor, bordering Iraq.
The SDF alliance of Kurdish and Arab militias has been focused on territory east of the river, which bisects the province.
The Syrian army, with Russian air power, has been waging its own separate offensive against Islamic State militants, mostly to the west of the river.
Al-Omar oil field lies some 10 km (6.21 miles) north of the town of al-Mayadin, which government forces took earlier this month.