At least 16 members of the Syrian Kurdish security forces were killed Wednesday in an Islamic State group attack on a base in the northeastern province of Hasakeh, a monitor said.
"There was a large explosion at dawn today in Hasakeh city caused by an Islamic State car bomb attack on a base belonging to the Kurdish internal security forces," the Syrian Observatory for Human Rights said.
The Britain-based group said IS jihadist militants opened fire on Kurdish forces after the blast, prompting fierce clashes.
All the attackers were killed, the Observatory said, without specifying the number of IS fighters involved in the assault in the provincial capital, where Kurdish and regime forces share control.
IS holds large areas of the countryside around Hasakeh and has attacked it on previous occasions.
Syrian state media reported a "terrorist car bomb" in Hasakeh city had left several dead and wounded, without giving a toll, and said the jihadists had also bombarded the city.
"IS terrorists targeted neighbourhoods (of Hasakeh city) with rockets and mortars," killing three people and wounding four more, the official news agency SANA said.
The Observatory said at least four people had been killed in the bombardment.
In the southern province of Quneitra, rebel fighters led by Al-Qaeda's Syrian affiliate Al-Nusra Front chased out IS-linked Jaysh al-Jihad from areas bordering the Israeli-occupied portion of the Golan Heights.
The Observatory said the nine-day battle had left at least 78 fighters dead in total, including 46 from Al-Nusra and its allies, and 32 from Jaysh al-Jihad.
He said the coalition of Al-Nusra, Islamist factions and local rebels had retaken control of the village of Al-Qahtaniyah and surrounding areas.
Clashes and fierce shelling in the area began on April 27, after Jaysh al-Jihad ambushed local rebels, killing six.
East of the Syrian capital, the Observatory said rebels had regained control of a town and a key road leading to the besieged anti-regime bastion of Eastern Ghouta.
"Regime forces collapsed and were forced to withdraw from the area," the Observatory said.
Syria's army on Sunday seized the village of Maydaa and cut off the nearby opposition supply route leading to Eastern Ghouta, but rebels regained both two days later.
"We retook Maydaa on Tuesday, and on Wednesday we took control over other areas previously under the control of the army," said Islam Alloush, spokesman for Jaysh al-Islam, the largest rebel group in the area.
"We are advancing," he said.
Syria's conflict began with peaceful protests in March 2011, but has since spiralled into a multi-front war that has left more than 220,000 people dead.