The video, uploaded by activists reporting on war crimes in Syria from the eastern province of Dair as-Saur, shows Denis Cuspert, whose rap names was Deso Dogg, with other fighters who are seen killing several people.
The jihadists kill three people during the video, with one shot and another beheaded.
Cuspert himself is not shown killing anyone, but briefly holds the severed head of one of the victims.
He tells the camera in German that the dead were opponents of Isis.
“That's why they've received the death sentence,” he says.
The video's authenticity could not be immediately verified.
The security services (Verfassungsschutz) say that Cuspert has achieved “a significant momentum for mobilization among correspondingly radicalized people in Germany”.
Having sworn allegiance to Isis leader Abu Bakr al-Baghdadi, Cuspert is charged with mobilizing Islamic radicals in Germany.
He has already appeared in several videos calling on young people to join Isis.
Originally from Berlin's Kreuzberg district, Cuspert converted to Islam in 2007 and becoming involved with radical groups.
Security services believe the 39-year-old has become a member of the Isis inner circle since travelling to the Iraq and Syria warzone from Egypt in April.
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