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A German rapper who became an Islamic State extremist apparently survived an October airstrike in Syria that the US military previously claimed had killed him, the Pentagon has acknowledged.

Denis Cuspert, also known by his artist name Deso Dogg, used to rap in Berlin and became one of the most famous Western fighters for IS.

He appeared in numerous IS propaganda videos, including one that US officials said pictured him showing a man's severed head.

The Pentagon last year declared Cuspert dead following an October 16 strike near the extremists' stronghold of Raqqa.

"At the time, our assessment was the strike was successful. It now appears that assessment was incorrect and Denis Cuspert survived the airstrike," Pentagon spokesman Major Adrian Rankine-Galloway said in a statement.

According to The Fader music magazine, which first reported the story, German intelligence officials had never conclusively been able to say Cuspert was dead.

The singer had appeared in other videos released by the IS group after the October strike, though he didn't explicitly refer to the attack.

Rankine-Galloway said he could not elaborate on why the United States now assessed Cuspert survived the strike.

"Because we are talking about intelligence, I really can't speak in great detail about this. However I can tell you that, at the time of the strike, our assessment was one of a successful strike against Denis Cuspert," he said.

Cuspert had pledged allegiance to IS leader Abu Bakr al-Baghdadi and was a chief recruiter of German fighters.

According to US officials, Cuspert had made threats against President Barack Obama and US and German citizens, and had also encouraged Western Muslims to carry out IS-inspired attacks.

IS prohibits music, but singing is allowed, and some of the jihadists' grisly videos are set to vocal sound tracks.