A German jihadist who fled Isis after witnessing beheadings and executions in Syria has claimed the group is trying to plan a Europe-wide terror attack.

The 27-year-old former militant, named as Harry S, said he and other foreign fighters had been asked if they would “bring jihad to their homeland”.

“They want something that happens everywhere at the same time,” he said.

Harry S was speaking to Der Spiegel from prison, where he is still being questioned by police and the intelligence services after being arrested at Bremen airport on his return in July.

He claimed he fled Isis because he could not stand its brutality after three months with the group in Syria and is now telling German authorities all he knows.

The former extremist appeared in a propaganda video filmed shortly after Isis seized the city of Palmyra in May.

Wearing camouflage, he carried the flag of the so-called Islamic State across the screen before German-speaking militants called on supporters across the world to kill “infidels” before shooting two prisoners dead.

“All you need is to take a big knife, and go down to the streets and slaughter every infidel you encounter,” they urged.

The video, entitled “The lions of the Caliphate: A message signed in blood to Angela Merkel” featured notorious Austrian Islamist Mohamed Mahmoud, who founded a banned Salafist group called Millatu Ibrahim.

Since travelling to join Isis in Syria, Harry S said he had been leading mass executions and holding weekly ideological training sessions in Raqqa.

Reports that Mahmoud and former Berlin rapper Denis Cuspert (aka Deso Dogg), who went under the name Abu Talha al-Almani, have been killed in air strikes have not been confirmed by the German government.

Harry S said he met both men for the first time in Syria, having apparently been radicalised after meeting German Islamist René Marc Sepac in prison as he served a two-year sentence for robbery.

After attempting to join Isis in 2014, when he was arrested and returned to Bremen by Turkish authorities, he had his passport confiscated and was ordered to check in at a police station twice a week, Der Spiegel reported.

But in the spring of his year he managed to travel under someone else’s passport to Syria, where he says he was trained to become part of a special unit intended to carry out urban combat missions before detonating suicide bombs. But he fled before being sent into battle.

Harry S stands accused of membership of a terror group and faces a lengthy prison sentence if convicted.

His lawyer, Udo Würtz, said his client did not directly take part in any atrocities, calling him a “lackey who allowed himself to be misled by the propaganda of Isis and who misled himself”.

Germany’s domestic intelligence agency believes more than 700 Germans have joined Isis in Iraq and Syria.