Islamic State militants killed 300 people in an “appalling massacre” in eastern Syria, the government said on Sunday.

The state-run SANA news agency said that most of those killed in day-long attacks on Deir el-Zour Saturday were elderly people, women and children. The killings are some of the worst carried out by the extremist group, which controls a large portion of Syria and Iraq.

The Syrian Observatory of Human Rights, a British-based group which monitors the violence on all sides of the Syrian conflict, confirmed the killings. Activists for the group said at least 135 were killed in the assault, at least 80 of them were soldiers and pro-government militiamen and the rest were civilians.

The group said that most of them were either shot dead or beheaded. The Lebanon-based Al-Mayadeen TV said ISIS took at least 400 civilians hostage.

A Syrian source told Reuters the terror group “committed a massacre among the civilians.” He said the pro-government army killed a large number of militants.

“They sent six suicide bombers first and they tried to break into military positions but failed,” the source told Reuters.

ISIS controls most of the province and provincial capital with the same name, while the Syrian government controls a few neighborhoods in the northern part of the city and the adjacent military airport. Most of the casualties took place in the area of Baghalijeh near the northern tip of the city

The ISIS-affiliated Aamaq news agency had reported a large-scale multi-pronged attack on Deir el-Zour that began with a suicide bombing. Opposition activists said Russian warplanes were carrying out intensive airstrikes in support of government forces in the area.

The government-held areas of the eastern Syrian city have been under siege by ISIS forces for over a year and more than 200,000 people there are living in horrid conditions and lack food and medicine.

The Associated Press contributed to this report.

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