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 Friday.

Most of Thursday's dead came south of the desert town of Sukna, 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, his first in months.

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.

Thursday's attacks targeted government forces around Deir Ezzor and on their supply lines through the Sukna 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 Sukna."

The attacks by the jihadists came as they face multiple offensives against the last bastions of their self-proclaimed caliphate -- by US-backed fighters and Russian-backed government forces in Syria and by troops and paramilitaries in Iraq.