The so-called Syrian Observatory for Human Rights said on Thursday that the ISIL militants took control of al-Omar oil field in Deir Ezzor Province, after al-Qaeda-linked al-Nusra Front militants withdrew without a fight.
According to sources, the Nusra Front militants pulled out of Mayadin and Shuhail towns on Thursday, leaving most of Deir Ezzor Province, which is close to Iraq's border, under the control of the ISIL militants.
The oil filed produced 30,000 barrels of crude a day before a crisis erupted in Syria in 2011. It fell to the Nusra Front last November.
The latest developments come as infighting has plagued militant groups fighting the Syrian government, as the army continues to gain more ground.
Clashes between the rival militant groups still continue despite an order by senior al-Qaeda leaders to stop the infighting.
More than 7,000 people have been killed since January in infighting among foreign-backed Takfiri militants who are wreaking havoc on Syria.
The Syrian government says its troops are going ahead with their operations to flush the foreign-backed militants out of residential areas across the country.
Over 160,000 people have reportedly been killed and millions displaced due to the violence fueled by the Western-backed militants.
SHI/SHI