A former London postman who joined Isis has been charged with involvement in a mass execution in Syria.

Harry Sarfo, who is already serving three years in a German prison for terror offences, was not accused of murder until footage of the massacre emerged last year.

The federal public prosecutor’s office said he was charged with six counts of murder and violating human rights law at a specialist state security court in Hamburg.

“In mid-June 2015 the so-called Islamic State had six prisoners executed on Palmyra’s market square,” a spokesperson said.

“Sarfo belonged to the six-member squad that carried out the execution and he was armed with a pistol.

“Together with other members of his group, he guarded the prisoners and prevented them from escaping.”

Harry Sarfo (top right) was seen pointing a gun towards captives during an Isis execution in Palmyra, Syria, in June 2015 (Washington Post)

Prosecutors said Sarfo led one of the captives to the middle of the street, where they were then shot, adding: “During the shooting, he stopped at the side of the road so as not to be hit by a bullet himself.

“From there, he took aim and fired at the bodies lying on the ground.”

In an interview with The Independent in January 2016, the 28-year-old said he never fought for the terror group during his three months in the “caliphate”.

While failing to mention his own involvement in the atrocity, Sarfo named his worst memory of Syria as the “execution of six men shot in the head by Kalashnikovs”, identifying it as one of the events that drove him to flee the terrorist group’s “barbarity”.

But Sarfo, who grew up in the UK after moving from Germany as a child, was caught on video herding captives to be executed in the Syrian city of Palmyra.

Footage of the massacre obtained by the Washington Post shows Sarfo with a group of Isis fighters led by Austrian Isis fighter Mohamed Mahmoud and German militant Yamin Abou-Zand.

He had already appeared in a propaganda video that showed the pair shooting Syrian captives dead in the ancient ruins of Palmyra, while calling on Isis supporters to travel to Isis territories or “kill infidels wherever you find them” in Europe.

Former London student in Isis execution video

In the second video cited by German prosecutors, which was not released by Isis’ propaganda agency, Sarfo is seen apparently herding one of six captives wearing combat fatigues with their hands bound into a public square in Palmyra.

Sarfo stands immobile by a wall for opening seconds of the fusillade, but he then pulls out a pistol and aims it at the men on the ground.

The camera is briefly obscured but Sarfo appears to fire towards unmoving victims. It is unclear whether a bullet hit and whether the captives were already dead.

German prosecutors said five of those killed were members of the Syrian army, while the sixth was a Sunni preacher condemned by Isis, which itself claims to represent Sunni Muslims.

Footage of the massacre was leaked by a source inside Isis, which is intensifying efforts to discredit defectors and featured Sarfo in a recent propaganda magazine decrying “fools who strayed” and spread “lies and falsehoods”.

Since being jailed he has spoken out against Isis’ ideology and said he wants to work with young men and women at risk of radicalisation.

“I've realised that what they are claiming to be Islamic is totally un-Islamic,” he told The Independent in an interview conducted via his lawyer from prison.

“I came to the conclusion that this is not the path to paradise, it is the path to hell.”

Palmyra recaptured by Syrian government forces Show all 10 1 /10 Palmyra recaptured by Syrian government forces Palmyra recaptured by Syrian government forces Palmyra recaptured by Syrian pro-government forces Graffiti on the ancient stones reads in Arabic ‘Shooting without the permission of the chief is prohibited’ Getty Palmyra recaptured by Syrian government forces Palmyra recaptured by Syrian pro-government forces Damaged artefacts lay inside the museum of the historic city of Palmyra Reuters Palmyra recaptured by Syrian government forces Palmyra recaptured by Syrian pro-government forces Syrian pro-government forces rest by Palmyra Citadel as they take control of the city from the hands of Isis Getty Palmyra recaptured by Syrian government forces Palmyra recaptured by Syrian pro-government forces The UNESCO world heritage site appears surprisingly intact after its recapture from the militant group Getty Palmyra recaptured by Syrian government forces Palmyra recaptured by Syrian pro-government forces Many had feared the ancient city would be destroyed following its capture by Isis in May Getty Palmyra recaptured by Syrian government forces Palmyra recaptured by Syrian pro-government forces Smoke billows from the Palmyra Citadel as Assad’s forces drive the Jihadist group from the city Getty Palmyra recaptured by Syrian government forces Palmyra recaptured by Syrian pro-government forces Palmyra is one of the ‘most important cultural centers of the world’ Unesco says Getty Palmyra recaptured by Syrian government forces Palmyra recaptured by Syrian pro-government forces Pro-government forces play football in the streets following the recapture of the city Getty Palmyra recaptured by Syrian government forces Palmyra recaptured by Syrian pro-government forces The extent of the destruction caused by Isis’ 10 month occupation of the city has yet to be fully realised Getty Palmyra recaptured by Syrian government forces Palmyra recaptured by Syrian pro-government forces The City Council of Palmyra building in ruins Reuters

Sarfo was sentenced to three years in prison for membership of a foreign terrorist organisation in July last year, having travelled to Syria in March 2015.

Prosecutors have also opened a separate case into accusations of war crimes, which continues.

Sarfo fled back to Germany in July 2015 and was immediately arrested upon his arrival at Bremen airport.

A German citizen of Ghanaian descent, he converted to Islam aged 20 in London, where he attended Leyton Sixth Form College and Newham College of Further Education.

He worked at Wickes and as a postman for Royal Mail before being sent back to Germany to serve a prison sentence for involvement in a 2010 armed robbery at a supermarket.

After being jailed with a known al-Qaeda recruiter, Sarfo said he “learned the ideology of jihad” and joined an extremist mosque after being freed, later deciding to join Isis after being repeatedly searched, detained and questioned by counter-terror police.

He said he trained in Isis’ special forces in its Syrian territories but fled the group before taking part in any operations, maintaining he did not kill anyone and refused to launch terror attacks in Europe.