The Syrian Democratic Forces (SDF), a Kurd-led coalition allied with the US, seized the Koniko field in Syria's eastern province of Deir Ezzor, according to the Syrian Observatory for Human Rights.
Islamic State militants were trying to recapture the field by carrying out counter-attacks, the watchdog added.
The SDF said in an online statement that its forces had seized Koniko after they had besieged Islamic State fighters for two days. SDF fighters are vying with Russia-backed government forces to capture of Deir Ezzor, an oil-rich region that has been under Islamic
State control since 2014.
In recent weeks, the SDF and the Syrian forces have each claimed gains against the radical group in of Deir Ezzor, which borders Iraq.
A general view of the eastern Syrian city of Deir Ezzor on September 20, 2017 as Syrian government forces continue to press forward with Russian air cover in the offensive against Islamic State group jihadists across the province. AFP
US-backed Syrian forces on Saturday captured the war-torn country's largest gas field from the Islamic State extremist militia organization, a monitoring group and Kurdish fighters said.