Four people were seriously injured in two knife attacks in one district of the Austrian capital Vienna on Wednesday evening but any motive or connection between the assaults was unclear, a police spokesman said.

The stabbings - the first of which took place at 6.45pm (GMT), occurred in Vienna's Second District, which includes the famous Prater park as well as the Praterstern transport hub. It also houses much of Vienna's small Jewish community.

"A man attacked a family - a father, a mother and their 17-year-old daughter - with a knife. The three people were seriously injured and their lives are in danger," the spokesman said.

"And half an hour later there was another attack ... at Praterstern and here a person was also seriously injured and their life is in danger. We do not know to what extent there is a connection here and any motive remains unclear," he said.