The French prime minister has put the country on its highest state of counter-terrorism alert after an assailant fatally stabbed a teacher and seriously injured two others.

Witnesses say the knifeman shouted “Allahu Akbar”, or “God is greatest”, during the attack at a school in Arras, northern France. He is now in custody.

The “attack emergency” level has been used in previous counter-terror cases.

The alert can trigger extra security deployments and public warnings.

The attack came amid rising tensions in France’s sizeable Muslim and Jewish communities due to the conflict between Israel and Hamas.

Interior Minister Gérald Darmanin said there was “no doubt” a link between the Arras attack and the Israel-Hamas conflict.

PM Elisabeth Borne took the urgent step after a security meeting with President Emmanuel Macron.

Earlier Mr Macron visited the Gambetta high school and condemned the “barbarity of Islamist terrorism”. He called on French people to stay “united” in the face of the attack, to “not give in to terror or let anything divide us”.

The man killed was a French language teacher, while another teacher and a security guard were injured.

Mr Macron said the teacher had “come forward to protect others and had without doubt saved many lives”.

The attacker, named as 20-year-old Russian national Mohamed Mogouchkov, is of Chechen origin and known to the security services for his involvement with Islamist extremism, according to police.

As a former pupil at the school, he alarmed teachers with his extremist language.