A host of never before seen work by the late great US artist Saul Leiter is being showcased at one of the world's biggest photography festivals in France.

Almost all of the paintings and drawings in the major Saul Leiter show at the "Rencontres d'Arles" festival in the southern French city have never been shown before.

A former fashion photographer, Leiter — who died in 2013 — was a leading light of the New York school of photography of the 1940s and 1950s that included Diane Arbus, Richard Avedon, and Robert Frank.

Curator Anne Morin called him "one of the 20th century's most fascinating artist-photographers".

"Everything is a matter of balance, exactitude, and humility in the works of this man, who nonetheless accorded great importance to imperfection," she said.

In a short film presented at the show, Leiter said "You don't have to apologize" for looking for beauty.

While summer temperatures outside soared well above 30 degrees Celsius (86 degrees Fahrenheit), the mercury hovered at around 20C in an underground exhibition space called the "cryptoportiques" where merchants once stored their wares in Roman times.

Fittingly, it is hosting images of prehistoric caves at Arcy-sur-Cure in central France taken by French photographer Juliette Agnel.

Drops of water occasionally oozed into the cryptoportiques, bringing to life the humidity pictured in the caves.

The photos had to be covered with a fine film to protect them from the moist conditions, festival director Christoph Wiesner told AFP.

Agnel illuminates only some parts of the caves in her photos, which show imaginary creatures and play on light and darkness in the manner of Old Masters like Rembrandt and Goya.

The work "explores extreme environments, whose unsettling beauty provokes senses of wonder and feelings of the sublime", wrote curator Marta Ponsa.