Investigators have said it could take days to identify all of the victims who died in the fire that tore through a crowded bar in the Swiss ski resort of Crans-Montana, as a local official said many of the injured were in a life-threatening condition.

About 40 people were killed in the blaze that engulfed the town’s Le Constellation bar, which was packed with mainly young revellers celebrating the new year, and about 115 others injured, many of them seriously, authorities have said.

Officials said it was too early to determine the cause of the blaze, but attention has focused increasingly on the ceiling of the bar’s basement, which witnesses and mobile phone images suggest may have been set alight by sparklers or flares attached to champagne bottles.

Social media footage shows person trying to put out flames in Crans-Montana bar – video

The head of the Valais regional government, Mathias Reynard, said experts were using dental and DNA samples in the “terrible and sensitive” task of identifying the badly burned bodies. “Nothing can be told to the families unless we are 100% sure,” he said.

Béatrice Pilloud, the Valais public prosecutor, said “significant resources” had been put in place to “identify the victims and return the bodies to families as fast as possible”. The canton’s police chief, Frédéric Gisler, said the process could take several days.

Stéphane Ganzer, a regional health and safety official, told RTL radio that several of the injured had not yet been identified, either because they were not carrying ID or because it had been lost in the fire, adding that many were also a critical condition.

“I think a large number of the injured, maybe between 80 and 100, are in a life-threatening condition,” Ganzer said on Friday. “When 15% or more of an adult’s body has third-degree burns, there is a risk of death in the days and hours that follow.”

A Swiss air ambulance coming in for landing at Sion airport after the fire in Crans-Montana. Photograph: Lisa Leutner/Reuters

The victims are believed to be of many nationalities. France’s foreign ministry has said nine French nationals were among the injured and eight were still missing. Italy has said six of its nationals are still missing and 13 were being treated in hospital. Australia has said one of its nationals was injured in the blaze.

The Italian foreign minister, Antonio Tajani, would visit Crans-Montana on Friday, the country’s ambassador to Switzerland, Gian Lorenzo Cornado, said. Cornado told Italian media that 47 people had been killed in the blaze, but Ganzer said on Friday he was “surprised” by that figure. “This is not the same number that we have,” he told RTL radio.

The EU said it had been in contact with Swiss authorities about providing medical assistance. France’s president, Emmanuel Macron, said some of the injured were being cared for in French hospitals. Others were hospitalised in Germany.

Several witness accounts broadcast by the Swiss, French and Italian media pointed to sparklers or flares placed in champagne bottles and held high by restaurant staff as part of a regular “show” for patrons, who made special orders to their tables.

There were “waitresses with champagne bottles and little sparklers. They got too close to the ceiling, and suddenly it all caught fire”, one witness, Axel. told the Italian media outlet Local Team.

Pilloud said investigators would be examining whether the bar met safety standards. Ganzer said images circulating on social media “seem fairly clear”, adding that there were similar flares and sparklers “in every disco in France, Switzerland and Europe”. He said the investigation would determine whether the material of the basement’s ceiling, which some images suggest was covered in a form of cladding, was safe.

Several sources told Agence France-Presse that the bar’s owners were two French nationals originally from Corsica. According to a relative, the couple are safe but have been unreachable since the fire.

Residents of Crans-Montana, many of whom knew victims, have been stunned by the disaster. Hundreds of people stood in silence near the scene as they came to pay their respects to the dead and injured on Thursday night.

On Friday, the mound of floral tributes outside Le Constellation bar continued to grow. “Rest in peace among the stars,” one of the messages read.

“I woke up to a loud bang at about 1.30am but then it went silent,” said François, who did not want to give his surname. “I fell back to sleep and then saw the news in the morning. It seems that so many young people have lost their lives. We’ve never experienced anything like this.”

Arlino Marchese and his friend Sacha Dimic, from the nearby town of Sierre, were in Crans-Montana to ski on Friday. “We used to go to Le Constellation a lot when we were younger,” said Dimic. “It was a good bar, with a good atmosphere and really popular. All those lives gone, it’s terrible.”

“They were people like us,” said Piermarco Pani, an 18-year-old who, like many others in the town, knew the bar well. Dozens of people left flowers or lit candles on a makeshift altar at the top of the road leading to the bar, which police had cordoned off.

Elisa Sousa, 17, told Reuters she was meant to have been at Le Constellation on Thursday night but had spent the evening at a family gathering instead. “I’ll need to thank my mother a hundred times for not letting me go,” she said at the vigil.

The Swiss president, Guy Parmelin, who visited the mountain resort on Thursday, said the country would hold five days of mourning to mark what he described as one of the most traumatic events in its history.