The Islamic State group killed at least 58 Syrian Government troops and militia as the jihadists put up fierce resistance to a Russian-backed offensive against some of its last bastions, a monitor said on Friday.

Most of yesterday’s dead came south of the desert town of Sukhna, east of the ancient city of Palmyra, the Syrian Observatory for Human Rights said.

IS put out a statement claiming to have killed scores of regime fighters in the area and also released what it said was an audio recording of its leader Abu Bakr al-Baghdadi urging resistance, the first such intervention in nearly a year.

Syrian troops pushed through the vast desert that separates the main cities of the west from the Euphrates Valley this summer and broke a three-year IS siege of government enclaves in the eastern city of Deir Ezzor earlier this month.

Yesterday’s attacks targeted government forces around Deir Ezzor and on their supply lines through the Sukhna area from the west, the Observatory said.

“The first attacks were carried out against checkpoints manned by loyalist troops in Al-Shula,” a village near Deir Ezzor, the Britain-based monitoring group said.

“IS then carried out a series of attacks against checkpoints along the length of the motorway from Al-Shula to south of Sukhna.”

Syrian state media made no mention of the army’s losses, but said its troops “confronted an attack by the terrorist Daesh group on the highway between Deir Ezzor and Palmyra, deep in the Badia desert.”