In a breaking news alert, state television gave an "initial toll" of 31 dead and 170 wounded, adding that rescue workers were still retrieving victims of the blasts.
The Syrian Observatory for Human Rights, a Britain-based monitor, earlier gave an initial toll of 14 dead in the devastating bombings.
Qamishli has regularly been targeted in bomb attacks, many of which have been claimed by the Islamic State group.
The IS-linked Amaq agency reported that a "suicide bomber in a truck bomb hit the headquarters of the Kurdish" forces in Qamishli, without explicitly claiming the attack as an IS operation.
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A source in the Kurdish Asayesh security forces told AFP "this is the largest explosion the city has even seen."
He added that local hospitals had been flooded with casualties from the attack.
Syrian state television meanwhile carried an appeal from the governor of Hasakeh province, where Qamishli is located, urging residents of the city to "go to public and private hospitals to donate blood for the victims of the terrorist bombings.