Marine Le Pen under investigation for posting IS execution pictures

French far-right leader Marine Le Pen has been placed under investigation after posting graphic images on Twitter of Islamic State (IS) group atrocities, including a beheading, in response to a journalist whom she accused of likening her party to IS.

The prosecutor's office in the western Paris suburb of Nanterre told the AFP news agency on Wednesday that it had launched an investigation into "the dissemination of violent images" over the National Front (FN) leader's series of posts.

In one image a bloodied body in an orange jumpsuit lies on the ground with a severed head on his back, another shows a man on fire in a cage, while a third shows a victim being driven over by a tank.

"THIS is Daesh!," said Le Pen, referring to IS in a post addressed to BFM TV journalist Jean-Jacques Bourdin. Le Pen also has more than 830,000 Twitter followers.

The images were still visible on her account Wednesday evening.

Prime Minister Manuel Valls described the photos as "monstrous".

"Madame Le Pen: inflaming public debate, political and moral failing, non-respect for victims," he wrote on his Twitter account.

Interior Minister Bernard Cazeneuve alerted the police to look into the tweets "as they do every time these photos are published".

The photos are "Daesh propaganda and are a disgrace, an abomination and an absolute insult to all victims of ... Daesh," said Cazeneuve.

Le Pen was reacting to comments by Bourdin whom she accused of drawing parallels between her party and the IS group in an "unacceptable bungle".

Bourdin, during his morning show known for combative one-on-one interviews, asked a question of Arab affairs expert Gilles Kepel in which he suggested there were "links" between FN and IS, with both seeking to push the French into cultural isolationism.

In later remarks Bourdin dismissed Le Pen's reaction as "hysterical".

"At no point did I say the FN was like Daesh," he insisted.

FN politician Gilbert Collard also posted a picture on Twitter of an IS victim, albeit one that was far less graphic than those posted by Le Pen.

"We are only showing the hate-filled ignominy of those who assimilate us with killers," Collard, who was also placed under investigation, said by way of explanation.

Le Pen's FN scored a record number of votes in regional elections on Sunday, boosted by concerns over the migrant crisis and terrorism, though the party failed to win control of any regions.

In the wake of the 13 November attacks in Paris that left 130 people dead, Le Pen warned that if IS was not conquered "Islamist totalitarianism will take power in our country".