A London postman who joined ISIS in Syria has been charged with taking part in the killing of six prisoners after footage showed him wielding a gun as they were shot dead.

Harry Sarfo can be seen with other jihadists herding captives into a market square in Palmyra ahead of the sickening public execution.

Footage also appears to show the former Royal Mail worker pointing his pistol towards the prisoners.

The 28-year-old, already serving three years in a German jail for terror offences, has now been charged with murder over the 2015 mass-killing.

Sarfo, who moved to London as a teenager, has previously said he trained with ISIS in Syria and experienced bombings 'nearly every day', while he also heard jihadis talking about plans to attack the UK and Europe.

Footage captures London postman Harry Sarfo (top right) wielding a gun during an ISIS execution in Syria

He was convicted last year by a Hamburg court of membership in a terrorist organisation. He was given three years in prison, with judges saying his comprehensive testimony had counted in his favor.

German prosecutors have now launched a new case after a video published by the Washington Post showed Sarfo appearing to take an active role in executions.

Prosecutors say he led one of the captives to the execution site and prevented others from fleeing.

The federal public prosecutor's office in Hamburg, Germany, said he was charged with six counts of murder and violating human rights law.

According to the Independent, a spokesman said: 'In mid-June 2015 the so-called Islamic State had six prisoners executed on Palmyra's market square,'

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

The 28-year-old (pictured), already serving three years in a German jail for terror offences, has now been charged with murder after being accused of participating in the killing of the six ISIS prisoners in 2015

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

'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 January it was reported that the jihadi had been charged with war crimes after footage emerged of him shouting ISIS slogans ahead of the terror group's public executions.

He had been lauded by a judge for a 'wholehearted' confession about his time as a member of ISIS, but then faced charges of concealing his true involvement which prosecutors believed extended to killing captives.

He was convicted in 2015 by a Hamburg court of membership in a terrorist organisation and breaching weapons export laws but was sentenced to just three years in prison when the judge looked favourably upon his detailed testimony.

Video, released by the Washington Post, shows him praying with fellow terrorists in a huddle, attempting to whip the crowd up by chanting ISIS slogans from the back of a truck and appearing to smile and laugh as prisoners are shot dead in the street.

At one stage, Sarfo seems to load his gun and point it in the direction of one of the captured men.

Sarfo, a former Royal Mail worker who moved to London as a teenager, has previously said he trained with ISIS in Syria and experienced bombings 'nearly every day', while he also heard jihadis talking about plans to attack the UK and Europe

Following his conviction last July, Sarfo gave several interviews from prison in which he portrayed himself as disillusioned and disgusted by ISIS.

Born to Ghanaian parents in the northern German city of Bremen, Sarfo converted to Islam after moving to London.

He became radicalised while in a German prison for taking part in a robbery and claims to have traveled to Syria in April 2015 to join ISIS.

He fled the group several weeks later and was arrested in July 2015 upon his return to Germany.

In August, he told authorities the terror group sent him away from Syria and back home to Germany telling him they wanted him to be part of a wave of attacks across Europe.

He had spent four days straight driving to Syria to join the extremists before spending three months with them in their Raqqa stronghold.

However, he was arrested when he left the caliphate and tried to return to Europe.

In an interview with the New York Times from his prison cell, Sarfo also revealed how ISIS did not want Europeans to travel to Syria anymore.

He told the newspaper that a commander within the terror group told him what they really wanted them to do was return to Europe to unleash a fresh wave of attacks.

Sarfo told authorities the terror group sent him away from Syria and back home to Germany telling him they wanted him to be part of a wave of attacks across Europe. File picture shows ISIS fanatics

Sarfo explained: 'He was speaking openly about the situation, saying that they have loads of people living in European countries and waiting for commands to attack the European people. And that was before the Brussels attacks, before the Paris attacks.

'They said, "Would you mind to go back to Germany, because that's what we need at the moment".

'And they always said they wanted to have something that is occurring in the same time.

'They want to have loads of attacks at the same time in England and Germany and France.'

Sarfo said he was attracted to the group because of its ideology of uniting people under one flag, and that he told its leaders he would not be prepared to take part in terror attacks.

He became radicalised after being forced to leave the UK in 2010 when he was jailed for a robbery in Germany.

While in prison he spent time with an Al Qaeda-linked recruiter and then joined a radical mosque in Bremen after his release.