About 40 people are believed to have been killed and 100 injured after a fire tore through a crowded bar during a New Year’s Eve party in the Swiss ski resort of Crans-Montana, according to the Italian foreign ministry.

Swiss police confirmed “several dozen” partygoers were dead. The victims could not be immediately identified because of the severity of their burns, the ministry said. It confirmed arson was not responsible, with the blaze thought to be the result of an accident.

“There has been an explosion of unknown origin,” Gaetan Lathion, a police spokesperson in Wallis canton in south-west Switzerland, told AFP. “There are several injured, and several dead.”

He said the blast took place at about 1.30am local time (0030 GMT) in a bar called Le Constellation, which is popular with tourists, as revellers rang in the new year. “More than a hundred people were in the building, and we are seeing many injured and many dead,” he said.

Video from the scene shows orange flames billowing from inside the ground-floor bar and lounge. Screams can be heard, as well as loud music. Other revellers stand in the snow outside the building, which is located in the centre of the Valais resort.

Speaking during a press conference on Thursday morning, Mathias Reynard, president of the canton of Valais, said what should have been a moment of celebration “turned into a nightmare”.

Police commander Frédéric Gisler spoke of “several dozen fatalities”, adding that he was devastated by the tragedy. “I can’t hide from you that we are all shaken by what happened overnight in Crans,” he told a press conference.

“Our count is about 100 injured, most seriously, and unfortunately tens of people are presumed dead,” he said, adding that patients had been dispatched to hospitals in Sion, Lausanne, Geneva and Zurich.

“At the moment we are considering this a fire and we are not considering the possibility of an attack,” prosecutor Beatrice Pilloud stressed, adding that authorities had opened a full investigation.

She said authorities were trying to get the bodies of the victims to their families. “A lot of resources have been put into forensics to identify the victims. These resources are intended to allow us to get the bodies to the families as soon as possible,” she said.

Some of the victims are from other countries, said Stéphane Ganzer, head of security for the Valais canton.