Scores of wounded were rushed to the city's hospitals, which appealed for blood donations, it said.
The Kurdish YPG, or People's Protection Units, said in a statement at least 20 people were killed and dozens injured, adding it was not clear whether the blast was caused by a explosive device or a suicide bomber.
Syrian state media said at least 30 were killed and 90 injured in the blast.
A statement by Amaq news agency, which is close to Islamic State militants, said a suicide bomber had attacked a gathering of Kurdish YPG fighters on the edge of the city with machineguns and an explosive vest. They did not give further details.
Earlier on October 3, two suicide bombers killed a number of people in the Syrian government-held city of Hama, the state news agency SANA reported.
The first blew himself up using an explosive belt in a square in the city's al-Hader district, followed 15 minutes later by a second in the same location, SANA reported, citing a police source.
Some unconfirmed reports said a woman suicide bomber may have been behind the Hasaka attack.
Hasaka is mainly in the hands of the YPG militia after it evicted the Syrian army in August.
The incident took place in early evening at a public hall on the highway between Hasaka and Kurdish-controlled Qamishli city, further northeast.
Islamic State militants have been driven out of most of Hasaka province, but the group continues to wage hit-and-run attacks on Kurdish forces. Washington considers the Kurds its main ally against the militants, arming and equipping them.