REPUBLIKA.CO.ID, BEIRUT -- 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, 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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