The group calling itself the Islamic State, previously known as the Islamic State of Iraq and Levant, has taken control of the rebel-held portion of the eastern Syrian city of Deir Ezzor, buoyed by advances in neighbouring Iraq, a monitoring group has said.

Rival rebel groups fighting against forces loyal to President Bashar al-Assad either changed sides or fled from the strategic Euphrates valley city, the Syrian Observatory for Human Rights said on Monday.

Assad forces remain in control of the other half of the city, a provincial capital and pre-war hub of the country's oil industry.

According to the Britain-based Observatory, which relies on a network of activists and medics on the ground, fighters from the Islamic State group were now in control of "95 to 98 percent of Deir Ezzor province".

The regime-controls half of Deir Ezzor city, a handful of villages as well as the military airport.

The Observatory said that rivals of the Islamic State group, including fighters of al-Qaeda's Syria affiliate, Jabhat al-Nusra, lost control after negotiations failed with the Islamic State group whose leadership last month declared a "caliphate" straddling Syria and Iraq.

"Fighters from Jabhat al-Nusra and the [Islamist] rebel movement Ahrar al-Sham withdrew from their bases in the city, while others pledged their loyalty to [the] Islamic State," the Observatory said.

"Islamic State killed the Deir Ezzor chief of al-Nusra and raised their flag in the city," it added.

Lack of rebel support

The rebel spokesman for Deir Ezzor confirmed the reports, blaming international backers of the anti-Assad opposition for a lack of support.

Speaking to the AFP news agency via the Internet, Omar Abu Leyla said: "The withdrawal is a result of the lack of any formal financial backing [for the rebels] either from the [exiled] opposition or from the international community."

The Islamic State group seized huge supplies of weapons and money in Iraq's second city Mosul last month in a lightning offensive that saw it capture a swathe of territory north and west of the capital, Baghdad.

Its gains in Iraq have tipped the balance in the struggle for power in rebel-held areas of eastern and northern Syria where it has been fighting armed groups allied with al-Nusra since January.

The Islamic State group already controls the city of Raqqa upstream from Deir Ezzor where it has enforced its hardline form of Islam, with public executions, including crucifixions.

Abu Leyla added: "Islamic State 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."

He berated international opposition backers for failing to adequately arm Deir Ezzor's rebels.