The state-run SANA news agency said that most of those killed in yesterday's attacks were elderly people, women and children while opposition activists said many of the victims were Syrian soldiers and pro-government militiamen and their families.

The killings are some of the worst carried out by the extremist group, which controls large parts of Syria and Iraq and has killed thousands of people in both countries.

The Britain-based Syrian Observatory for Human Rights, which documents all sides of the Syria conflict through activists on the ground, said late yesterday that at least 135 people were killed. It said around 80 of them were soldiers and pro-government militiamen and the rest civilians.

It added that many of them were shot dead or beheaded. The Islamic State group controls most of Deir el-Zour province and much of the capital with the same name, while the government controls several districts in the northern part of the city and the adjacent military airport.

Most of the casualties took place in the area of Baghaliyeh near the city.

The Lebanon-based Al-Mayadeen TV, which is close to the Syrian government, also reported a massacre and said IS killed dozens of people, including women and children, and threw their bodies in the Euphrates River. It said the group took more than 400 civilians hostage.

The reports could not be independently confirmed. The IS-affiliated Aamaq news agency had reported a large-scale multi-pronged attack on Deir el-Zour that began with a suicide bombing.

Today, it reported that the group expanded its control of areas west and northwest of Deir el-Zour, adding that around 110 Syrian government forces were killed and at least five others captured. It said the group seized control of Baghaliyeh and surrounding areas during the fighting.

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