Syria army and fighters from the Kurdish People's Protection Units (YPG) launched uncoordinated offensives against ISIS in the northeastern province of Hasakeh, the Syrian Observatory for Human Rights said.
Observatory head Rami Abdel Rahman told AFP that after three days of clashes, syria forces bolstered by fighters from Arab tribes had secured control over 23 villages in the centre of the province from ISIS.
Syria's official news agency SANA put the number at 31.
State television said the army offensive would continue until it controlled the main road linking the provincial capital Hasakeh and the city of Qamishli.
"ISIS has launched counter-attacks on Syrian army checkpoints, while the army fortifies its positions with support from local Arab tribes," Abdel Rahman added.
He said YPG fighters were meanwhile also battling ISIS alongside Arab tribes outside the village of Tal Tamr in Hasakeh's southwest.
"The YPG fighters in Tal Tamr are shelling ISIS around the area to lure ISIS to respond, so they can identify their positions" and call for strikes by the US-led coalition waging an air campaign against IS, he said.
"But ISIS is avoiding any response in order not to give away its positions."
YPG spokesman Redur Khalil confirmed to AFP that the Kurdish fighters were conducting "attack-and-retreat operations with IS on two fronts.
Around Tal Burak," a town between Hasakeh and Qamishli, he said.
ISIS launched an attack last week on the areas around Kurdish-controlled Tal Tamr and kidnapped 220 Assyrian Christians from 11 villages. Nineteen of them were freed Sunday after ransoms were paid.
Control of Hasakeh province is split between ISIS, Syrian army and Kurdish militia, with overlap at a number of points.
The area is of strategic importance because it borders both Turkey and Iraq.
menwhile a UN fact-finding mission deployed to Syria's second city Aleppo on Monday, despite the rejection by opposition forces of a partial ceasefire there proposed by UN envoy Staffan de Mistura.
"The mission will aim to assess the situation on the ground and to ensure that, once the freeze is announced, humanitarian aid can significantly increase, and to prepare arrangements to follow up on violations of the freeze," his office said in a statement.
The Italian-Swedish diplomat has made the Aleppo freeze the centrepiece of his mediation efforts since he was named as special envoy on the Syrian conflict in July.