NANTERRE (AFP) - Four group leaders at a French summer camp are under investigation after a series of photos were discovered of them mimicking hostage executions and acts of torture, local authorities said Friday.

A man on his knees, a knife to his throat, surrounded by "jailers" in balaclavas, is pictured in one of about 20 images uncovered by the municipality of Colombes, just outside Paris, which organised the camp.

"We filed a complaint on Thursday. They have also been suspended from their jobs and disciplinary action is under way," said Colombes mayor Nicole Goueta.

The photos were taken during the camp in August on an island in Brittany in northwest France.

The municipality was putting together an exhibition of images from summer camps when they stumbled upon the pictures on a USB key.

It is unclear whether any of the 45 children at the camp, aged between eight and 12, were involved in the staged scenes.

One of the pictures showed a child sitting in a corner looking scared but it was unclear whether "he was witnessing a horrific scene or if he was being punished," said Goueta.

"It is now up to the judicial system to find out if these men are susceptible to radical Islam."

The revelation of the photos comes as France grapples with homegrown radicalisation, with several nationals seen in videos released by the Islamic State insurgent group in Iraq and Syria.