BEIRUT — Syria’s government said Sunday that Islamic State militants killed 300 people in an “appalling massacre” committed in the eastern city of Deir el-Zour as the extremists made significant advances in the contested region.

The state-run SANA news agency said most of those killed in Saturday’s daylong attacks were elderly people, women and children, but 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 Syrian conflict through activists on the ground, said late Saturday that at least 135 people were killed, many of them shot or beheaded. It said around 80 of them were soldiers and pro-government militiamen and the rest were civilians.

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.

The Lebanon-based Al-Mayadeen TV, which is close to the Syrian government, also reported a massacre and said ISIS 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 reported Sunday that the group expanded its control of areas west and northwest of Deir el-Zour, seizing control of Baghaliyeh and surrounding areas during the fighting.