An offensive by the Islamic State militant group in eastern Syria has left at least 45 pro-regime fighters dead, a monitoring group said Tuesday.
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 jihadists.
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 Shiite militias 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 jihadists are holding out in remote parts of the Euphrates Valley.