"With support from Russian aviation, regime forces entered Mayadeen and took control of several buildings in the west of the town," Rami Abdel Rahman, head of the Britain- based Syrian Observatory for Human Rights, told AFP.
IS jihadists have controlled Mayadeen in the eastern province of Deir Ezzor since 2014.
President Bashar al-Assad's forces have been advancing rapidly in a bid to expel the jihadists from the province.
Mayadeen sits on the western bank of the Euphrates River, between provincial capital Deir Ezzor, where the jihadists still hold several districts, and the border with Iraq.
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