The jihadist Islamic State (IS) now fully controls all of Syria's main oil and gas fields in Deir Ezzor province bordering Iraq, a monitoring group said on Friday.
The IS has declared an "Islamic caliphate" in areas it controls in Syria and Iraq, where it is spearheading an offensive against government forces.
"IS took control of the Tanak oilfield, located in the Sheiytat desert area in the east of Deir Ezzor province," late on Thursday after rival rebels withdrew, the Syrian Observatory for Human Rights said.
Earlier the same day, the jihadists seized the major Al-Omar oilfield.
They have still not captured the tiny Al-Ward 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.
The IS seized Tanak and Al-Omar after rival fighters from the Al-Qaeda-linked Al-Nusra Front and other Syrian rebel groups withdrew, said the Observatory.
In Deir Ezzor, which holds the bulk of Syria's oil resources, the IS has taken over nearly all the countryside, its forces bolstered by heavy weapons captured from Iraqi troops fleeing the offensive that it headed.
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 gone on the counter-offensive in the northern city.
On Friday, it seized the Kurdish villages of Zur Maghar and Bayada near the border with Turkey, the Observatory said.
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- Intense air raids -
Abdel Rahman told AFP that Aleppo city's rebel-held areas "are now surrounded from all sides, by the regime and by the jihadists they are fighting".
Aleppo's rebel areas, 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 Turkey.
The Observatory said Friday that regime forces have made advances this week on the outskirts of eastern Aleppo, threatening rebel supply routes.
State television, meanwhile, said that government forces were now in full control of the Sheikh Najjar industrial sector of northeast Aleppo.
Last 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".
Elsewhere in Syria, the regime air force kept up its air raids Friday against rebel areas, mainly in the southern province of Daraa, Aleppo, Damascus province and Idlib.
Warplanes also hit the Arsal area just across the border in neighbouring Lebanon, that country's National News Agency reported, killing one Syrian youth and wounding several more.
The area houses more than 60,000 Syrians who have fled the conflict in their homeland.
The UN children's fund, UNICEF, said Friday more than six million Syrian children affected by the conflict desperately need humanitarian aid, with the number in need rising by a third in a year.
Syria's war began in March 2011 as a peaceful movement demanding President Bashar al-Assad's ouster, but it has 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 an eventual regional conflagration.