Islamic State militants in Syria
Militant Islamist fighters ride horses as they take part in a military parade along the streets of Syria's northern Raqqa province Reuters

Sunni militants from the Islamic State (IS) control all the main oil and gas fields in Syria's eastern Deir Ezzor province which borders Iraq, according to a monitoring group.

"IS took control of the Tanak oil field, located in the Sheiyat desert area in the east of Deir Ezzor province," late on Thursday following a withdrawal by a competing rebel group, the opposition-aligned Syrian Observatory for Human Rights said.

IS, formerly known as Isis, the Islamic State of Iraq and al-Sham, had previously seized control of the major Al-Omar oil field but smaller oil facilities remain in the hands of local tribesmen, the Observatory's director said, as quoted by AFP.

IS, which recently declared the establishment of an Islamic caliphate spanning Syria and Iraq, has proliferated in eastern Syria at the expense of other rebel groups including the al-Qaeda linked Jabhat al-Nusra.

Nusra and other groups have aligned to fight against IS in Syria, after the group engaged in a notably brutal campaign against its enemies. While IS was driven from Syrian territory around Aleppo and in the province of Idlib, it has retained control of swathes of land in Raqqa province as well as Deir Ezzor.

IS recently launched a lightening campaign across the Syrian border in Iraq, where it seized the country's second city of Mosul after government-aligned security forces fled. The group then allied with a number of Sunni militants to sweep southwards, capturing towns and cities on the route to Baghdad as well as Iraq's biggest oil refinery at Baiji.

While the group's advance through Iraq appears to have been checked by government-backed resistance, IS remains in charge of a number of revenue generating energy facilities across the two countries. Selling oil provides a vital economic lifeline for the group, which has been branded a terrorist organisation by the United States.