A man has been "assassinated" in an Islamist terror attack in the Paris suburbs "because he was a teacher", France's president has said.

The history teacher, who is said to have discussed images of the Prophet Muhammad with his pupils, was beheaded, it has been reported.

The suspected attacker was shot dead about 600 metres from the scene, the AP news agency has said.

It happened on a street in Conflans Sainte-Honorine, northwest of the French capital, at about 5pm local time.

Visiting the scene, Emmanuel Macron called it a "cowardly attack", saying that the man was killed because he "taught the freedom of expression, of believing and not believing".

The president said France would "protect and defend" its teachers.

According to Le Parisien newspaper, the attacker was an 18-year-old man of Chechen origin, who was carrying a knife. A gun was found at his side. Other reports say it was an airsoft gun that fired plastic pellets.

The suspect was shot after he refused to put down his gun, Le Parisien reported.

The country's anti-terror prosecutor earlier called the incident a stabbing, but both the Reuters and AP news agencies said police sources told them the victim was decapitated.

Witnesses heard the attacker shout Allahu Akbar, or God is greater, Reuters said.

The teacher had reportedly shown images of the Prophet Muhammad in class during a discussion about freedom of expression about 10 days ago.

The suspect did not have a child at the school, AP said.

A Twitter thread posted last Friday alleged pupils had been shown cartoons of the prophet.

Le Parisien said the victim was "horribly mutilated".

Education minister Jean-Michel Blanquer tweeted that the republic had been attacked through the "despicable assassination of one of its servants".

He added that "unity and firmness are the only answers to the monstrosity of Islamist terrorism".

It is the second terrorism-related incident since the opening of an ongoing trial on the newsroom massacre in January 2015 at the satirical newspaper Charlie Hebdo after the publication of caricatures of the prophet of Islam.

As the trial opened, the paper republished caricatures of the prophet to underscore the right of freedom of expression.

Exactly three weeks ago, a young man from Pakistan was arrested after stabbing, outside the newspaper's former offices, two people who suffered non life-threatening injuries.

The 18-year-old told police he was upset about the publication of the caricatures.