The man who killed a schoolteacher in northern France last week was charged with terror offenses on Tuesday, a prosecutor said.

Mohammed Moguchkov, 20, was charged by an anti-terror judge with murder linked to a terrorist conspiracy and for associating with terrorist criminals. The suspect had swore allegiance to the Islamic State extremist group in an audio recording before stabbing Dominique Bernard, 57, at his former school in Arras, about 180 kilometers north of Paris, prosecutor Jean Francois Ricard told reporters in Paris.

Moguchkov, who was remanded in custody, had made a "very marginal" reference to Gaza in a video following the attack on Israel by Hamas, a source familiar with the case said.

The suspect's 16-year-old brother also appeared before the judge, suspected of having "provided him with some support."

The teenager was charged and placed in pre-trial detention, his lawyer Ambroise Vienet-Legue told Agence France-Presse.

Their 15-year-old cousin was also charged on suspicion of having been "informed of the plan" without doing anything to prevent it, according to Ricard.

The cousin is subject to a provisional judicial educational intervention, the National Anti-Terrorism Prosecutor's Office that said overnight Tuesday.

French President Emmanuel Macron told reporters in the Albanian capital, Tirana, on Tuesday that "Islamist terrorism" had returned.

France raised its security level after Friday's attack and deployed 7,000 troops.