BEIRUT: The Islamic State of Iraq and Al Sham (ISIS), now known as the 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.

The ISIS declared an “Islamic caliphate” in areas it controls in Syria and Iraq, where it is spearheading an offensive against government troops.

“The group took control of the Tanak oil field, located in the Sheiytat desert area in the east of Deir Ezzor province,” late Thursday after rival rebels withdrew, said the Syrian Observatory for Human Rights.

Earlier that day, ISIS 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 and is in the hands of a local tribe, said Observatory director Rami Abdel Rahman.

ISIS 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 ISIS as they swept across Syria imposing their hegemony and brutal abuse.

The rebels expelled ISIS from the northeastern Idlib province and from much of Aleppo, but the group remains firmly in control of its bastion in Raqa province.

In Deir Ezzor, ISIS has taken over nearly all the countryside, its troops bolstered by heavy weapons captured from Iraqi troops fleeing an offensive spearheaded by militants.

On Sunday, ISIS declared a “caliphate”, referring to an Islamic system of rule that was abolished nearly 100 years ago in a move which rebels including groups in Syria branded a “heresy“.

Syria's war began as a peaceful movement demanding Assad's ouster, but morphed into a conflict after a brutal crackdown by the regime.

Many months into the fighting, militants started to pour into Syria, drawing warnings from analysts of a looming regional conflagration.