Fighters also reportedly seized control of a checkpoint, a fire station, and university accommodation in Deir al-Zor, not far from Syria's eastern border with Iraq.
State television claimed government forces have fought their way into the historic Syrian city against Islamic State militants, backed by Russian air cover.
The Syrian Observatory for Human Rights said the death toll from a recent wave of Russian and Syrian airstrikes has risen to 164 over the past 72 hours.
The airstrikes reportedly targeted the Islamic State's de facto capital Raqqa and areas in the eastern province of Deir al-Zor, where an estimated 400 civilians were recently kidnapped.
The Syrian Observatory for Human Rights, a monitoring group, said that IS militants committed a mass abduction after attacking areas held by Bashar al-Assad's regime in the province of Deir al-Zor on Saturday.
A monitoring group says the militants committed a mass kidnapping after attacking areas held by the Assad regime in the province of Deir al-Zor.