French Prime Minister Elisabeth Borne has put the country on its highest security alert after an assailant fatally knifed a teacher and injured three other people in the northern French city of Arras.

The attack happened around 11:00 a.m. local time (0900 GMT) on Friday in Gambetta High School.

"In the current context and following the terrorist attack in Arras, I have decided to raise the security (preparedness) to emergency level," Borne posted on social media.

Speaking to the French public television TF1, French Minister of Interior Gerald Darmanin said earlier on Friday that the government would mobilize "a few thousand more" security personnel to monitor places where the public gathers.

Concerning the suspect who carried out the knife attack, Darmanin said the perpetrator is about 20 years old and had been flagged by French security services for radicalization.

"We suspected something," the minister told TF1, adding that the man's phone had been tapped for several days prior to the incident, but there was no indication of a planned attack.

Jean-Francois Ricard, public prosecutor covering anti-terrorism cases, told a press conference on Friday that the assailant is a former student of the school.

The attacker first stabbed a teacher who was in front of the school, then another person who came to help the teacher, Ricard said. Then after entering the building, the perpetrator injured two more people, both working in the school.