At least two people were killed and seven wounded in a knife attack on a regional train in northern Germany on Wednesday in which fellow passengers overpowered the alleged assailant, police said.

The stabbings occurred on a train travelling between the cities of Hamburg and Kiel, a police spokesman said, adding that three of the wounded had serious injuries.

The 33-year-old suspect, a stateless man of Palestinian origin, was detained at the railway station in the town of Brokstedt and treated in hospital for minor injuries.

"Witnesses were able to restrain the suspect immediately after the attack until police arrived at the station in Brokstedt," police said in a statement.

The spokesman said the investigation into a motive was focused on "all directions" including possible extremism or psychological problems on the part of the assailant.

Federal interior minister Nancy Faeser called the attack "shocking news" and said her thoughts were with the victims "of this terrible act" and their families.

Sabine Suetterlin-Waack, the interior minister for Schleswig-Holstein state, said federal and regional police were "working closely together" to determine a motive.

"For me it is clear that this horrific act was against any humanity," she said, adding that she was heading to the scene of the crime.

Police and emergency workers established a wide security perimeter around the Brokstedt station while helicopters circled overhead.

The daily Bild said the suspect had wounds on both hands when he was detained.

Germany's national rail company Deutsche Bahn said some trains on the line between Hamburg and Kiel had been cancelled to allow police to conduct their investigation.

Germany has been hit by several deadly knife attacks in recent years, some carried out by extremists and others by people suffering serious psychological problems.

A Syrian jihadist was given a life sentence in May 2021 for stabbing a German man to death and severely wounding his partner in a homophobic attack in the eastern city of Dresden.

Last June, a 30-year-old woman died from her injuries after an apparently random knife attack on students at a university campus.

In September 2022, a knife-wielding man wounded two people in Ansbach, a Bavarian town close to Nuremberg, before being fatally shot by police who said they were investigating a possible "Islamist or terrorist context".

A German court in December sentenced a Syrian-born Islamist to 14 years in prison for a knife attack on a train in which he injured four passengers.

Last year, a German court also committed a Somali to a psychiatric hospital after he stabbed three people to death in the southern city of Wuerzburg in 2021. (AFP)