The Britain-based Syrian Observatory for Human Rights said IS fighters seized Ein Issa and nearby areas before noon yesterday.
The fall of the town described as being "liberated" on IS-associated outlets comes after the main Kurdish militia known as the People's Protection Units, or YPG, captured wide areas in northern Syrian from the extremists, including the border town of Tal Abyad, once a main point for IS trade and smuggling in of foreign fighters.
Earlier yesterday, the Islamic State released a new video purporting to show the killing of two Syrian men in the militants' stronghold of Raqqa, allegedly for spying on the group.
IS has killed hundreds of people since it declared an Islamic caliphate in June last year with the city of Raqqa as its de facto capital.
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The video, which was posted on an IS-linked Facebook page, shows two young men in orange jumpsuits, saying they filmed and photographed IS-held areas in Syria and sent footage to a person abroad.
The video could not be independently verified but it was posted on a page known to be linked to the militant group.