Three Islamic State (ISIS) group car bombs killed 12 Syrian troops and militiamen in the northeastern city of Hasakeh where the jihadists have taken two districts, a monitor said on Monday.
The Sunday blasts came as troops battle to evict the jihadists from the two southern neighborhoods they seized on Thursday.
"At least 12 regime soldiers and allied fighters were killed in the blasts," the Syrian Observatory for Human Rights said.
Nine ISIS fighters were killed in clashes with regime forces on Sunday, it added. The fighting was continuing on Monday.
State news agency SANA said government forces destroyed an ISIS truck bomb, killing or wounding "a number of terrorists."
According to the United Nations, 60,000 people have been displaced by the fighting in the city, a provincial capital which had a pre-war population of 300,000.
The army has brought in reinforcements and has prevented the jihadists from advancing further but has struggled to push them back.
The Observatory reported fresh air strikes against ISIS positions in the city on Monday, and jihadist shelling of government-controlled neighborhoods.
Kurdish forces, which control several northern districts of the city, have so far restricted their involvement in the fighting to the defense of their own neighborhoods.
ISIS has sought repeatedly to enter Hasakeh, including last month when it advanced to the southern outskirts before being pushed back in heavy fighting.
ISIS has captured around 50% of Syria's territory since it emerged in the country in 2013.
AFP contributed to this report.