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

"Based on an eyewitness’ statements, a man has stabbed three people directly outside the restaurant behind me. All three victims have life-threatening injuries and are currently in a hospital," Police spokesperson Patrick Maierhofer said.

"What we know is that the attacker is a man. Eyewitnesses have told us he had a knife with him. He escaped in the direction of the street Praterstern. This is what we currently know. The manhunt for the criminal is still ongoing, and it's our highest priority," he added.

"There was a second stabbing attack soon after this attack, near Praterstern train station. We do not know yet if these two attacks relate to each other. We are investigating this issue," Maierhofer said.