The kidnappings come months into major battles for control of several parts of northern Syria that have pitted Kurdish fighters against al-Qaeda-linked terrorists, chiefly the Islamic State of Iraq and the Levant (ISIL).
"In the past three days, ISIL has kidnapped at least 51 Kurds in the towns of Minbej and Jarablus," said the so-called Syrian Observatory for Human Rights.
Among the hostages were nine children and a woman, said the Britain-based group, adding that there was no information on where they had been taken.
Minbej and Jarablus are located in Aleppo province, which is home to a Kurdish minority.
The kidnappings come weeks after Kurdish fighters further east, in majority Kurdish areas, expelled terrorists after battles that lasted several months.
In response, the militants have imposed a siege on Kurdish areas of Aleppo, where Kurdish fighters are weaker, said Observatory director Rami Abdel Rahman.
In July, ISIL kidnapped some 200 Kurdish civilians from the Kurdish towns of Tal Aran and Tal Hassel also in Aleppo province. Only a few of those hostages have since been released, the Observatory says.
BA/BA