Updated 11.45

A man wielding an axe and a knife has been shot dead after injuring at least four people in a rampage on a train outside the German city of Wuerzburg.

Three of the victims are in a serious condition, while another suffered minor injuries, a police spokesman Fabien Hench said. Another 14 people were treated for shock.

An eyewitness who lives next to the railway station told DPA news agency the train carriage, which had been carrying around 25 people, looked "like a slaughterhouse" with blood covering the floor and seats.

The man, who declined to give his name, said he saw people crawl from the carriage and ask for a first-aid kit as other victims lay on the floor inside.

Another witness told German newspaper Bild that they had never seen "so much blood" before.

Among the injured were four members of a family from Hong Kong, according to the South China Morning Post. They are a 62-year-old man, 58-year-old woman, their 27-year-old daughter and her 31-year-old boyfriend, the paper said.

The attacker appears to have been a 17-year-old refugee from Afghanistan, according to Bavaria's interior minister Joachim Herrmann. 

He said officials were still investigating the motive for the attack, however a handmade Islamic State flag was found among the belongings of the teenager.

BBC News reports that a text written in Pashtun was also found and it showed a strong indication that the teenager "could be a person who had been self-radicalised".

Herrmann said those who had interacted with the young man in recent months described him as calm and quiet and they could not understand his actions.

Islamic State also since released a statement claiming the attacker was one of its fighters.

The attack took place between Ochsenfurt and Wuerzburg-Heidingsfeld train stations in Bavaria at around 9.15pm local time. The train was brought to a halt during the attack when one passenger pulled the emergency brake.