Ibrahim Abdul Qader, 20, and his friend Fares Hamadi were found beheaded at home Friday morning.
The acctivist group documents abuses in areas under ISIS control in Syria.
"Turkey has long been accused by Syrian opposition activists, Kurdish fighters and sometimes even Western partners of allowing ISIS members to slip back and forth across its 911-kilometre (566-mile) frontier with Syria."
Several prominent ISIS accounts on social media have claimed the vile terrorist group was responsible for the horrific assassination in Turkey.
Sanliurfa is 55 kilometres (35 miles) from Turkey's border with Syria's Raqa province, a major ISIS stronghold in the country.
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The beheaded bodies of a Syrian activist opposed to the ISIS Terrorist group and a friend were found early Friday in the southern Turkish city of Sanliurfa, his colleagues said.