"There are now 45 dead from last night's attacks in Hasakeh, as most of those who were in critical condition passed away," the Syrian Observatory for Human Rights said, after reporting 33 killed last night.
Two explosions struck as members of Syria's Kurdish minority celebrated on the eve of Nowruz, the Kurdish new year, in the northeastern city of Hasakeh.
There was no immediate claim of responsibility yesterday, but Rami Abdel Rahman of the Britain-based Observatory told AFP the extremist IS group was behind the attacks.
The Kurdish People's Protection Units (YPG) control Hasakeh city, but IS militants have launched attacks on towns nearby.
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The jihadist offensive has sparked fierce clashes with Kurdish militia in the province, which is strategically located along Syria's borders with Iraq and Turkey.
UN chief Ban Ki-moon condemned the "heinous" attacks, while the commander of the Kurdish security forces, Joan Ibrahim, said "the crime that occurred today in Hasakeh will not pass without retribution."