An offensive by the "Islamic State of Iraq and Syria" (ISIS) in eastern Syria has left at least 45 pro-regime fighters dead, AFP reported Tuesday (June 5th).

ISIS 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 ISIS elements.

Observatory head Rami Abdel Rahman said at least 26 ISIS 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 militias present in the area, including groups from Iraq, Afghanistan and Lebanon's Hizbullah.