BEIRUT (AFP) - The Islamic State in Iraq and Syria (ISIS) group on Tuesday (July 19) released a video purportedly featuring a 17-year-old Afghan refugee who went on an axe rampage on a German train, injuring five people, two critically.

The video showed teenager "Mohammed Riyadh" - knife in hand - announcing in Pashto he would carry out an "operation" in Germany, and presenting himself as a "soldier of the caliphate".

Four members of a family of tourists from Hong Kong and a passer-by were hurt in the assault late Monday in southern Germany that appeared likely to rekindle tensions over the country's refugee influx.

German authorities said they had found a hand-painted ISIS flag and what they called a suicide letter among the asylum seeker's belongings. The assailant was killed by police as he attacked officers while trying to flee.

"The perpetrator of the stabbing attack in Germany was one of the fighters of the Islamic State," the ISIS-linked Amaq news agency said.

Amaq later released a video it claimed showed the attacker threatening "infidel" countries.

German authorities said they had authenticated the video.

Locals described the assailant, identified in media reports as Riaz A., as "calm and even-keeled" and a "devout Muslim who did not appear to be radical or a fanatic", according to Joachim Herrmann, interior minister of Bavaria state.

"According to the investigation thus far, there was no evidence on site to point to him belonging to the Islamist network," Herrmann said.

Police however later found a farewell letter he apparently left for his father, who still lives in Afghanistan, in which he said the world's Muslims "must defend themselves".

The teen had learnt on Saturday that a friend had died in Afghanistan, which police suggested could have pushed the attacker to act.

"Now pray for me that I can take revenge on non-believers, pray for me that I can get to heaven," the note said.

Prosecutors said he shouted "Allahu akbar" (God is greatest) three times as he made his way through the carriage.

An eyewitness told DPA news agency that the train, which had been carrying around 25 people, looked "like a slaughterhouse" with blood covering the floor.

Germany has thus far escaped the kind of large-scale Islamic militant attack seen in the southern French city of Nice last week, in which 31-year-old Mohamed Lahouaiej Bouhlel used a truck to mow down 84 people.

That attack was also claimed by ISIS without the assailant having clear ties to the group.

A record 1.1 million people were let in to Germany last year, with Syrians making up the largest group followed by Afghans.

The assailant had arrived as an unaccompanied minor in Germany in June 2015 and had been staying with a foster family in the region for the last two weeks, Herrmann said.

"We must determine what the motive was and to what extent he really belonged to the Islamist scene or self-radicalised very recently," Herrmann said, adding that the assailant had no criminal record in Germany.

In May, a mentally unstable 27-year-old man wielding a knife killed one person and injured three others on another Bavarian regional train.

Early reports had suggested he had yelled "Allahu akbar" but police later said there was no evidence pointing to a political motive. He is being held in a psychiatric hospital.

In February, a 15-year-old girl of Turkish origin stabbed a policeman in the neck with a kitchen knife at Hanover train station in what prosecutors later said was an ISIS-inspired attack.

And police in April arrested two 16-year-olds over an explosion that wounded three people at a Sikh temple, in what was believed to be an Islamist-motivated assault against an Indian wedding party in the western city of Essen.

Bavaria is governed by the Christian Social Union, a sister party to Chancellor Angela Merkel's conservative Christian Democrats which had been vocal in criticising Merkel's welcoming stance toward refugees.

The number of refugees arriving in Germany has fallen sharply as a result of the closure of the Balkans migration route and an EU deal with Turkey to stem the flow.

Merkel's popularity has rebounded recently as a result but the Bavaria attack is likely to rekindle political tensions.

Herrmann however warned against tarring all asylum seekers with the same brush.

"It is undisputed that he was a refugee and if he hadn't been there, he wouldn't have committed this act. But I don't think that we should make blanket judgements in any way about refugees."