Syrian opposition activists say Islamic State militants have killed dozens of government forces in wide-scale attacks on government-held areas of the eastern city of Deir el-Zour.

The opposition activist group the Syrian Observatory for Human Rights says at least 75 soldiers and pro-government militiamen were killed in the attacks Saturday which saw the group make significant advances in the contested city. It said at least 60 of them were shot dead by the extremists.

The Islamic State group controls most of the province and provincial capital with the same name, while the 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 Baghaliyeh.