"Film maker Naji Jerf, father of two children, was assassinated... today in Gaziantep", on the border with Syria, with a silencer-equipped pistol, the group said in a statement on Twitter.
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RBSS is a group of citizen journalists who work to expose human rights abuses in Raqa, the northeastern city that Islamic State uses as its de facto capital in Syria.
A friend of Jerf's said he had been "supposed to arrive in Paris this week after receiving, along with his family, a visa for asylum in France".
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Jerf was also editor in chief of Hentah, a Syrian magazine that reports on the "daily lives of Syrian citizens", said the publication's website.
Turkish media reported that the 37-year-old had been producing a documentary on massacres carried out by Islamic State militants when he was killed.
"He was hit by a bullet in the head as he was walking in the street and taken to hospital, where he died," the T24 news website reported.
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This is not the first time a Syrian occupation figure has been murdered in Turkey.
At the end of October, Islamic State claimed responsibility for killing militant Ibrahim Abdelkader and a friend. They were found decapitated in a house in Sanliurfa in southern Turkey.
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