The managers of the Swiss ski resort bar where dozens died during a fire at a New Year's celebration are under investigation for negligent manslaughter, authorities said in a statement on Saturday.

French couple Jacques and Jessica Moretti owned and managed Le Constellation bar in Crans-Montana, which was crammed with young partygoers when a blaze began in a packed basement at around 1.30am on Thursday.

"They are charged with manslaughter by negligence, bodily harm by negligence and arson by negligence," the statement said.

The question of whether safety standards were respected has been debated ever since the devastating blaze.

Jacques Moretti insisted to the Swiss press on Friday that all safety norms were followed at the bar, which according to the Crans-Montana website had a capacity of 300 people plus 40 on its terrace.

But even before charges were brought against the couple, chief prosecutor Beatrice Pilloud said safety compliance was a key focus of the investigation.

She told reporters on Friday the leading hypothesis was that "sparklers or Bengal candles attached to champagne bottles had lifted too close to the ceiling" and had ignited the blaze.

One video shared on social media showed the low wooden ceiling – covered with soundproofing foam – catching alight and the flames spreading quickly, as revellers continued to dance.

Forty people were killed and 119 injured, most of them seriously, according to the latest toll.

Crans-Montana is a popular destination for international tourists, and many foreign nationals, including a large number of French and Italians, are known to be among those hurt in the blaze. (AFP)