A German Isis deserter who claimed to have been appalled by the violence committed by the jihadist group was caught on video cheering a mass execution and apparently shooting a prisoner.

Former Isis militant Harry Sarfo, 28, is currently imprisoned in Germany on a reduced sentence, after publicly denouncing the group he spent several months with in 2015.

He claims he became disillusioned with Isis (Daesh) and deserted them after witnessing appalling violence carried out by militants.

"I witnessed stonings, beheadings, shootings, hands chopped off and many other things," he said in an interview before the trial, reported Deutsche Welle.

However footage published by the Washington Post seems to tell a different story.

In the footage Sarfo shuffles bound and blindfolded prisoners into a huddle in Palmyra, then leads the crowd in chanting and cheering as the prisoners are shot dead. He apparently shoots at one of the prisoners as he lies on the ground.

The footage was provided to the newspaper by an Isis insider, with the group keen to discredit Sarfo for having denounced them. Sarfo had previously claimed in media interviews not to have refused to have taken part in the execution while with Isis. He was filmed carrying the Isis flag in a propaganda video, but was not shown taking part in killings.

A German security official told the newspaper the emergence of the footage would probably lead to new charges being brought against Sarfo.

Before his arrest, Harry S was known to police in Bremen as a petty criminal who had committed thefts and robberies. He converted to Islam after living in London, and claimed to have been radicalised in jail in 2012.