An ultra-violent Islamist group has expelled moderate rebels from Syria's eastern city of Deir Ezzor, according to a Syrian opposition-aligned watchdog.

Militants from the Islamic State have strengthened their hold on the eastern oil-rich province of Deir Ezzor, which borders Iraqi territory they also control.

While Deir Ezzor remains peripheral to the Syrian conflict, its oil resources make it a valuable prize for rebel groups that have competed for control over the past year.

The Islamic State, formerly known as Isis (Islamic State of Iraq and the Levant,) recently captured Syria's largest oilfield, al-Omar. The group now controls all of Syria's major oilfields and the rebel-held half of Deir Ezzor.

The Islamic State, 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.

The hardliners expelled dozens of rebels from Deir Ezzor city, according to the rebel-aligned Syrian Observatory for Human Rights.

"The Islamic State is now in control of the entire Deir al-Zor province apart from a few areas and the military airport that the government is in control of," Observatory director Rami Abdurrahman said.

The majority of rebels either fled or pledged allegiance to the militants, while government forces are still in control of around half of the city.

Since the hardliners declared the establishment of an Islamic caliphate, some moderate rebels aligned with the Free Syrian Army have fled northern Syria to areas close to the Jordanian and Lebanese borders.

The influx of weapons from Iraq, where the extremists captured government arms, has empowered the militants, a spokesman for the rebels told AFP news agency.

"Isis has no shortage of weapons, ammunition or fighters, and the battle became totally asymmetrical, especially after its advance on Mosul and its capture of heavy weapons," rebel spokesman Omar Abu Leyla said, as quoted by AFP.