The fire in the Le Constellation bar in central Crans-Montana took a violent turn during New Year's celebrations.
Around 40 people have died in the fire, according to Swiss police.
In addition, 115 people were 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.
The country's president, Guy Parmelin, called the incident "one of the worst tragedies ever in our country."
The work to identify the dead is ongoing. People of several different nationalities are reported to have been affected by the incident. The Swedish Ministry of Foreign Affairs said 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 it is not being investigated as a deliberate attack.
Witnesses described an explosive fire, saying 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 among young people. The venue consists of two floors, with a staircase leading down from the upper floor to a basement room, local newspaper Le Nouvellist writes.
