IS declared an "Islamic caliphate" in areas it controls in Syria and Iraq, where it is spearheading an offensive against government troops.
"IS took control of the Tanak oil field, located in the Sheiytat desert area in the east of Deir Ezzor province," late yesterday after rival rebels withdrew, said the Syrian Observatory for Human Rights.
Earlier that day the jihadists seized the major Al-Omar oil field.
They have still not captured the tiny Al-Ward oil field, which produces barely 200 barrels of oil per day and is in the hands of a local tribe, Observatory director Rami Abdel Rahman said.
In Deir Ezzor, IS has taken over nearly all the countryside, its forces bolstered by heavy weapons captured from Iraqi troops fleeing an offensive that it spearheaded.
In January, Al-Nusra and other Islamist militants turned their guns on the jihadists, then known as the Islamic State of Iraq and the Levant, as they swept across Syria imposing their hegemony and brutal abuse.
The rebels expelled IS from the northeastern Idlib province and much of Aleppo, though the jihadist group has recently gone on the counter-offensive in Aleppo.
Speaking to AFP, Abdel Rahman warned that Aleppo city's rebel areas "are now surrounded from all sides, by the regime and by the jihadists they are fighting."
Aleppo's rebel areas, which lie mainly in the east of the city, have come under intense, daily air raids since December, leading tens of thousands of residents to flee for the countryside and to neighbouring Turkey.
On Sunday, IS declared a "caliphate," referring to an Islamic system of rule that was abolished nearly 100 years ago, in a move that rebels, including Islamists in Syria, branded a "heresy."