The jihadist Islamic State (IS) is now in full control of all main oil and gas fields in Syria's Deir Ezzor province, bordering Iraq, a monitoring group said.
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," after rival rebels withdrew, said the Syrian Observatory for Human Rights.
Earlier that day the jihadists seized the major Al-Omar oil field.
But they have still not captured the tiny Al-Ward oil field which produces barely 200 barrels of oil per day.
It is in the hands of a local tribe, said Observatory director Rami Abdel Rahman.
Islamic State seized Tanak and Al-Omar after rival fighters from Al-Qaeda's Al-Nusra Front and other Syrian rebel groups withdrew from those areas, said the Observatory.
In January, Al-Nusra and other Islamist militants turned their guns on the jihadists, then known as the Islamic State of Iraq and Syria, as they swept across Syria imposing their hegemony and brutal abuse.
The rebels expelled IS from the northeastern Idlib province and from much of Aleppo, but the jihadist group remains firmly in control of its bastion in Raqa province.
In Deir Ezzor, IS has taken over nearly all the countryside, its troops bolstered by heavy weapons captured from Iraqi troops fleeing an offensive spearheaded by Sunni militants.
Last Sunday, IS declared a "caliphate", referring to an Islamic system of rule that was abolished nearly 100 years ago in a move which rebels including Islamist groups in Syria branded a "heresy."
Syria's war began as a peaceful movement demanding Mr Assad's ouster, but morphed into a conflict after a brutal crackdown by the regime.
Many months into the fighting, jihadists started to pour into Syria, drawing warnings from analysts of a looming regional conflagration.
Iraq's incumbent prime minister, Nuri al-Maliki, has vowed not to drop his quest for a third term in power amid a growing chorus of critics urging that he be replaced in the next government.