The Al Qaeda splinter group seized al-Omar oil field, the biggest oil field in Syria's eastern province of Deir al-Zour, after the Al Qaeda-linked Nusra Front withdrew from the field, Xinhua quoted the UK-based Syrian Observatory for Human Rights as saying in a report.
ISIS and Nusra Front have been fighting for control over areas in Deir al-Zour, where large parcels of land in that oil-rich province have spiralled out of the government control.
The al-Omar field constitutes of a gas plant and a satiation for generating electricity. The field, with a production capacity of 75,000 oil barrels a day, was captured by the rebels last November after clashes with the Syrian military.
ISIS seized al-Omar, the biggest oil field in province of Deir al-Zour
ISIS seized the plant just two days after the ultra-jihadist organisation captured the key town of Bukamal in Deir al-Zour on the borders with Iraq. The group has also advanced into other nearby towns after other jihadist groups pledged alliance to it.
ISIS is the richest jihadist organisation in the world, according to a recent report by the pan-Arab al-Mayadeen TV. ISIS's budget equals that of small countries, as it now controls the majority of the oil fields in eastern and northern Syria.
Aside from dues their loyalists pay them regularly, ISIS also looted a total of $425 million from banks in Iraq's al-Musul city after storming it last month.
The report said that the organisation has been receiving funds over the past 10 years from rich people and organisations in Qatar, Saudi Arabia and Kuwait. ISIS's overall wealth exceeds $2 billion.
The new victory of ISIS comes just a couple of days after the organisation declared to establish an Islamic state in areas under its control in Syria and Iraq.
The Observatory said that more than 7,000 rebels and civilians have been killed as a result of the rebel-on-rebel fighting since earlier this year.
The Al Qaeda breakaway group now controls areas that stretch from Nineveh and part of al-Anbar region in western Iraq to the northern countryside of Syria's northwestern province of Aleppo, bordering Turkey.
Its control also stretch from northern countryside of Deir al-Zour to the south of al-Hasaka province, in addition to large areas of the eastern countryside of Deir al-Zour and the entire western countryside of that oil-rich province toward the al-Raqqa province, the group's main base in Syria.
Led by Abu Bakr al-Baghdadi, who commands tens of thousands of fighters, the group said that its goal is to establish an Islamic state in Iraq and Syria and to fight the Shiite government of Iraqi Prime Minister Nouri al-Maliki and that of Syrian President Bashar al-Assad, whose top ranks are from the Alawite minority, an offshoot of Shiite Islam.