ISIS had lost ground Saturday in a town on the Syrian-Iraqi border after pro-regime forces repelled a major attack the day before, a Britain-based monitor said.
Violent clashes were ongoing in Albu Kamal, which lies in the Euphrates Valley in eastern Syria, the Syrian Observatory for Human Rights (SOHR) monitoring group said.
"ISIS has retreated from inside the town to its western and northwestern parts" after pro-regime forces pushed back the militants, Observatory head Rami Abdel Rahman said.
Pro-government reinforcements had arrived, he said.
On Friday, ISIS used at least 10 suicide bombers in its offensive on Albu Kamal, swiftly taking several of its neighbourhoods, the monitor said.
It was the biggest attack on the town since the militant group lost it in November 2017, and the latest in a string of attacks by ISIS across Syria.
Neither Syrian state media nor the army reported Friday's attack on Albu Kamal.
The fighting in the town over the past 24 hours has killed 30 pro-regime fighters, SOHR said.
These include 16 regime troops among them a general, as well as 14 non-Syrian combatants, notably Iranians and Lebanese Hezbollah movement fighters, the monitor said.
Twenty-one ISIS militants were also killed in that same period, including the 10 suicide bombers, according to the same source.
ISIS has ramped up its attacks against pro-regime forces since its fighters in May left their last bastion near Damascus under an evacuation deal with the regime.
ISIS in 2014 proclaimed a cross-border "caliphate" in Syria and neighbouring Iraq, but has since lost most of its territory to various military offensives.
Its presence is mostly confined to pockets of eastern Syria in the vast desert stretching from the country's centre to the border with Iraq.
Earlier this week, ISIS assaults in the eastern province of Deir Ezzor killed 45 pro-regime fighters.
On Thursday, ISIS militants also killed 22 pro-regime fighters in surprise attacks in the southern province of Sweida, a monitor said.