Update 14 August 7am

Six people were injured in an attack aboard a Swiss train on Saturday by a man armed with a knife and flammable fluid, St Gallen cantonal police said.

Three adults, two teenagers and a child aged six were hurt, according to a police statement. Five were stabbed or burnt. A sixth suffered smoke inhalation as he came to the aid of the others. The assailant, a Swiss citizen, was also injured.

"According to current information, a 27-year-old Swiss man poured out a flammable liquid. He was also armed with at least one knife. The liquid caught fire," the statement said.

Authorities did not yet know the motive for the attacks but a crime of passion was one possibility, Bruno Metzger of the St Gallen cantonal police said in an interview with welt.de.

"A terrorism background still seems very, very far-fetched," Metzger said.

The assailant was in police custody, he said, but none of those injured - including the assailant - was responsive, so they could not be questioned.

The police said a criminal inquiry was under way.

Both a woman and the attacker were in critical condition, according to Swiss broadcaster SRF, citing the St Gallen police. The cantonal police did not respond to a request for further information by email and could not be reached by phone.

The attack occurred at 2.20pm (1220 GMT) as the train neared the Salez station on a trip between Buchs and Sennwald in St. Gallen, a canton in the northeast of the Alpine republic, the police statement said.

It was the first attack of its kind in Switzerland in recent memory. In December 2015, a 33-year-old Swiss man broke into his neighbours' house in Rupperswil and killed four people before setting the house on fire, an incident that shocked the country.