At least 16 members of the Syrian Kurdish security forces were killed on Wednesday in an IS militant group attack on a base in the northeastern province of Hasakeh, a monitor said.
The Britain-based group said IS 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 militants 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.
“IS terrorists targeted neighbourhoods (of Hasakeh city) with rockets and mortars,” killing three people and wounding four more, the official news agency SANA said.
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.
Last month insurgents including Al Nusra Front, captured the town of Jisr Al Shughour in Syria’s Idlib province, edging closer to the government-held heartland of Latakia along the coast.
The conflict has killed 220,000 people and displaced millions since 2011, despite earlier diplomatic efforts by de Mistura’s predecessors – Kofi Annan and Lakhdar Brahimi.