The Islamic State of Iraq and the Levant (ISIL) group has been accused of carrying out a massacre in Syrian government-held districts in the eastern city of Deir Az Zor. A monitoring group has said more than 130 people were killed.

The UK-based Syrian Observatory for Human Rights said on Saturday that at least 80 of those killed were pro-government militiamen and the rest were civilians, as the armed group captured the northern suburb of al-Baghaliyeh.

The Observatory added that at least 400 civilians, including families of pro-government fighters, were abducted.

"There is genuine fear for their lives, there is a fear that the group might execute them as it has done before in other areas," said the Observatory's head Rami Abdurrahman.

The state news agency SANA said ISIL fighters committed "a massacre", killing "around 300 civilians, most of them women, children, and elderly people" in Baghaliyeh.

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The reports could not be independently confirmed.

The advance in the northern tip of Deir Az Zor puts ISIL in control of about 60 percent of the city with the regime holding the rest, according to the observatory, which monitors violence in Syria through a wide network of local sources.

The monitor also said Russian warplanes were carrying out heavy air strikes in support of Syrian forces as they sought to repel the fighters.

Deir Az Zor is the capital of the oil-rich province with the same name.

The province links ISIL's de facto capital in the Syrian city of Raqqa with territory controlled by the group in neighbouring Iraq.

The Syrian conflict started as a largely peaceful uprising against President Bashar al-Assad in March 2011, but morphed into a full-blown civil war that has killed more than 250,000 people and turned about 4.3 million others into refugees, according to the UN.