UPDATE 1-Oslo stabbing probed as terrorist attack - police

A knife attack in a supermarket in Oslo on Thursday is being investigated as a possible act of terrorism, the head of the Norwegian police's security service (PST) said on Friday.

A woman was knifed as she was paying at the till and the suspect, a 20-year-old Russian man, was arrested soon after, police said.

"The man said under questioning that he wanted to kill several people and that this was an act of terror," PST head Benedicte Bjoernland told a news conference.

Police were investigating possible links to Islamist extremism, she said, adding that the victim was critically ill in hospital. (Reporting by Gwladys Fouche, editing by Terje Solsvik and John Stonestreet)