French Foreign Minister Laurent Fabius said Monday that air strikes in Syria must target Islamic State militants but also other groups "considered as terrorists."

Fabius said a statement by President Francois Hollande on Friday that Russian air strikes must target "Daesh and only Daesh (the Arabic acronym for IS)", did not exclude other groups like the Al-Nusra Front.

"Of course, it is a concise formulation, it is Daesh and groups considered as terrorists," Fabius told Europe 1 radio in an interview, referring to Hollande's statement.

Moscow, which has launched more than 70 air strikes in Syria since last Wednesday, has come under fire for targeting Western-backed moderate opposition and IS fighters alike in their bid to bolster President Bashar Al-Assad.

President Barack Obama has called Russia's dramatic intervention a "recipe for disaster", while British Prime Minister David Cameron urged Russian President Vladimir Putin on Sunday to "change direction" in Syria and recognise that Assad must be replaced.

Fabius warned of the risk that the Syrian conflict could turn into a wider religious war.

"When you see a conflict which at first was a civil war, becoming a regional war involving international powers, Russia, Iran, the US, the risks are serious," he said.

"The most terrifying risk is that the conflict becomes religious: If you have Shiite (Muslim) populations on one side with their allies, and Sunni populations on the other side with their allies, it is an inferno which can be extremely dangerous."

Assad is an Alawite, an offshoot of Shiite Islam, who is supported by regional Shiite heavyweight Iran as well as Hizbullah.

On the other side Sunni powers like Saudi Arabia and Qatar are opposed to Assad and have backed the Islamist groups fighting him. They are also taking part in a U.S.-led coalition carrying out an air campaign against IS.

France, which has been targeting IS in Iraq for the past year, began striking the group in Syria eight days ago.