The fire in the Le Constellation bar in central Crans-Montana turned violent during New Year's celebrations.
Around 40 people are believed to have died, according to Italy's Foreign Ministry, which cites Swiss police.
More than 100 people are believed to have been injured, several of them seriously. According to public service broadcaster SRF, at least 30 injured people have been flown by air ambulance to hospitals in Lausanne, Geneva and Zurich, among others. Some have reportedly been taken to a children's hospital and at least one person has also been flown to Stuttgart in Germany.
Identifying the dead is ongoing. People of several nationalities are reported to have been affected by the incident. The Swedish Ministry of Foreign Affairs says the embassy in Bern is in continuous contact with local authorities and is trying to obtain more information about the incident.
The fire was reported at 1:30 a.m. It is still unclear exactly how the fire started, but authorities say it is not being investigated as a deliberate attack.
Witnesses describe an explosive fire and say the wooden interior of the bar caught fire after small fireworks were dropped into empty champagne bottles. SRF reports that the fire quickly spread from the wooden ceiling to "all flammable surfaces" in the premises.
The bar Le Constellation is described as a popular spot for young people. The venue has two floors, with a staircase leading down from the upper floor to a basement room, writes local newspaper Le Nouvelliste.
