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The militant group itself has not reported the death of the founder of Aamaq news agency, Baraa Kadek.
But his brother, Hozaifa, posted on his Facebook that Kadek and his young daughter were killed in an airstrike in Mayadeen town in Deir el-Zour province.
Other Syrian opposition news outlets, including Halab News Network and Qasioun news agency, reported Kadek's death late Wednesday. But it was not immediately clear where Kadek was killed or when. His brother said he was killed in his home.
Halab News Network, an activist-run media platform where Kadek previously worked, said he was killed with his daughter in his car. All said he was killed Wednesday in a suspected airstrike by the U.S-led international coalition fighting ISIS in Syria and Iraq.
There was no immediate comment from the coalition. Last week, the coalition told The Associated Press it had carried out a series of airstrikes on May 25 and 26 targeting ISIS media infrastructure and "propaganda facilities." It said at the time that targeting such facilities "degrades" the group's abilities and inspiration of foreign attacks.
Aamaq news agency has become the group's fastest and most reliable source of information, using it to post videos and claims of attacks. It has remained online despite bans from social media platforms.
Mayadeen has become a refuge for ISIS leaders as the group comes under attack in Mosul in Iraq and their de-facto capital Raqqa. Some Syria watchers said the group's media operations have moved to Mayadeen as the U.S.-led coalition and allied Syrian Kurdish-led forces close in on Raqqa.
Kadek became known as a media activist covering the outbreak of protests in 2011 then rebel fronts. Originally from Aleppo province, he later joined ISIS in 2013 and set up the group's flagship media arm.