PARIS (AFP) - The firm operating the Paris subway has come under fire for apparently pushing France's beloved secularity too far with its refusal to display an ad with the mention "Christians of the East".

French music group "The Priests" had planned to advertise their upcoming June concert in Paris with a poster sporting a banner that said proceeds would go towards the cause of Christians persecuted in places such as Iraq and Syria.

But the state-owned RATP firm that operates the subway and its advertising agency Metrobus ordered the group's producers to take off the banner, pointing in a statement issued last week to the public service's requirement for neutrality "in the context of an armed conflict abroad".

But Jean-Michel Di Falco Leandri, the bishop who created "The Priests", told AFP that Metrobus had previously explained the ban on the banner was due to "the religious nature of the mention 'Christians of the East'," and the ad was published without it.

France is a deeply secular country where the state and religion are strictly separated.

But the RATP's censorship move has nevertheless created a stir, with some netizens pointing out that while mentioning "Christians" appears to be banned, the subway operator allowed an ad promoting an extra-marital affairs website.

"One must not confuse secularity, which we respect, with secular extremism which becomes a grotesque and ridiculous witch hunt," Di Falco Leandri said.

CHREDO, a French group that aims to raise awareness of the plight of Christians in areas such as the Middle East - where they are currently being persecuted by jihadist groups - has since asked a court to rule on the controversy and a hearing will take place on Wednesday.

France's upper house Senate, meanwhile, has also announced it will audition those at the RATP responsible for the decision not to run the ads with the banner.