Three teenagers were indicted Friday in the murder of French history teacher Samuel Paty, beheaded last month after showing cartoons of the Prophet Mohamed to his students during a free speech lesson, a judicial source told AFP.

Two 18-year-old men and a 17-year-old girl have been charged with "criminal terrorist conspiracy", the source said. They were arrested on Tuesday.

The men -- one French and one a Russian of Chechen origin -- are suspected of having been in contact with Paty's killer, 18-year-old Abdullakh Anzorov, and have been placed in pre-trial detention.

The girl was placed in a youth detention centre. She had been in contact with one of the two men.

Paty, 47, was decapitated near his school in a Paris suburb by Anzorov, also a Chechen, in response to a social media campaign denouncing the teacher's use of cartoons published by satirical weekly Charlie Hebdo in a class on freedom of expression.

His murder sparked a torrent of outrage that prompted President Emmanuel Macron to crack down on Islamist extremism and violence in a country reeling from a wave of jihadist attacks since 2015 that have killed more than 250 people.

Anzorov was shot and killed by police.

Ten people have now been charged in the case, including a 14-year-old and 15-year-old accused of pointing Paty out to his killer.

The three latest suspects were detained in separate regions of eastern France, a source said, hundreds of kilometres from the Paris suburb of Conflans-Sainte-Honorine where Paty was killed, and far from the Normandy city of Evreux where Anzorov lived.

Also charged in the case is a known Islamist radical who helped an outraged father of one of Paty's students stir up a campaign against him on social media.

The father has also been charged, as have three others accused of helping Paty carry out the killing.

Being charged in France does not necessarily mean a suspect will end up on trial, as a case can still be dropped for lack of evidence.

- 'Supporting terrorism' -

Also Friday, prosecutors in the northern city of Cambrai said three teenagers between the ages of 15 and 17 were charged with "supporting terrorism" for threats made during a national tribute to Paty on Monday.

Similar incidents were reported in several French cities where youngsters suggested Paty had got what he deserved for showing the cartoons, viewed as offensive by many Muslims.

Education Minister Jean-Michel Blanquer told RTL radio Friday that about 400 such incidents had been reported during a minute of silence observed for Paty at schools countrywide, "some expressed mildly, others more forcefully".

Legal action is likely in about a dozen more serious cases, Blanquer said.

Macron's vociferous defence of French freedom of speech in response to Paty's murder has sparked furious protests in several Muslim-majority countries.

Charlie Hebdo republished the caricatures in September to mark the start of a trial for 14 suspects accused of complicity in the massacre of 12 people, including cartoonists, at the paper's Paris offices by two jihadist gunmen in January 2015.

An Algerian man was convicted this week by a court in Pontoise northwest of Paris to six months in prison for "glorifying" Paty's murder on social media and describing the killer as a "martyr", judicial sources said Friday.

He was also banned from France for 10 years, which means he will be sent back to Algeria after serving his sentence.