The clashes come with the army pressing an advance through Raqa, in northern Syria, towards neighbouring Deir Ezzor, the only remaining province of the war-ravaged country still in the hands of ISIS terrorists.
The Syrian Observatory for Human Rights, a Britain-based monitor, said the fighting had claimed the lives of 38 terrorists and 26 pro-regime combatants since Tuesday morning.
It takes to 145 the overall deaths in six days of fighting in villages on the banks of the Euphrates River in the east of Raqa province, near Deir Ezzor.
ISIS said in a statement on Tuesday that its members had killed dozens of regime combatants in "intense fighting lasting hours".
Observatory director Rami Abdel Rahman told AFP that the regime was fighting to secure a foothold in Raqa province "in order to advance in Deir Ezzor".
The jihadists have laid seige to government forces and civilians in the provincial capital of Deir Ezzor since 2015.
Earlier this month, government troops and allied fighters arrived at the outskirts of Madan, the last ISIS-held town in the countryside of eastern Raqa province before Deir Ezzor.
But ISIS launched a counterattack last week that pushed the regime forces back, and fighting has since continued.
The Syria army operation in the area, backed by air support from ally Russia, is separate from the battle for provincial capital Raqa city.
The effort to oust ISIS from the city is being led by the Syrian Democratic Forces, a US-backed alliance of Kurdish and Arab fighters.