PARIS - French President Emmanuel Macron said on Saturday a knife attack that killed one and injured three in eastern France on Saturday was "Islamist terrorism", after France's anti-terrorism prosecutor's office confirmed it was investigating the case.

A man attacked local police officers in the city of Mulhouse shouting "Allahu Akbar" ("God is greatest") on Saturday afternoon, the PNAT prosecutor's office said in a statement.

A passer-by was killed trying to intervene, while three police officers were injured, the prosecutor's office added.

"It is without any doubt an act of Islamist terrorism," Macron told reporters on the sidelines of the annual French farm show, adding that the interior minister was on his way to Mulhouse.

The suspect has been arrested, the prosecutor's office said.

Reuters