IS fighters launched the operation Sunday against Euphrates Valley villages seized last year by government forces and their allies, and have retaken four of them, the Syrian Observatory for Human Rights said.
The villages are located on the road between the provincial capital of Deir Ezzor and the city of Albu Kamal, which lies further south on the border with Iraq.
Near those villages is the small town of Hajin, the single largest populated hub still under the control of IS.
Syria not for Syrians
Observatory head Rami Abdel Rahman said at least 26 IS fighters were also killed since "fierce" fighting started on Sunday.
He said there had not yet been any intervention by Russian aircraft supporting pro-government forces in the area.
The Observatory said the casualties on the pro-regime side were mostly fighters from Shia militants present in the area, including groups from Iraq, Afghanistan and Lebanon's Hezbollah.
The small pockets controlled by IS in that area are the last dregs of the sprawling self-styled caliphate the group proclaimed over large parts of Syria and Iraq in 2014.
A series of victories by Russian-backed regime troops and US-backed Kurdish forces have shattered the caliphate, but thousands of surviving are holding out in remote parts of the Euphrates Valley.
COMMENTS
Comments are moderated and generally will be posted if they are on-topic and not abusive.
For more information, please see our Comments FAQ