About 40 people were killed and 115 injured in a blaze in at a New Year's Eve Party at an upmarket Swiss ski resort, with the disaster likened to scenes from a horror film.

This article contains details some readers may find distressing.

Most of the victims of the fire in Crans-Montana are thought to be young people, but identification of the bodies is expected to take several days at least.

The fire broke out at roughly 1.30am local time in the Constellation Bar as people celebrated the turn of the year.

Read more:
What we know about deadly Swiss fire
People 'walking on top of others' trying to escape

Emergency services were on the scene minutes later, but it was already too late to save dozens of people, thought to have been trapped in the basement.

Authorities warned that naming the victims or establishing a definitive death toll would take time due to the severity of the burns. Experts have been using dental and DNA records to try to identify the dead.

Switzerland's President Guy Parmelin described the deadly incident as "one of the worst tragedies our country has ever known", and King Charles called it a "horrific disaster".

The country will hold five days of mourning.

The cause of the blaze remains unclear, but witnesses reported seeing sparklers or candles in bottles in the basement bar.

But Beatrice Pilloud, Valais Canton attorney general, said it was too early to determine the cause of the fire. Experts have not yet been able to go inside the charred wreckage of the bar.

'Horror movie' scenes of people with hair burned off

Witnesses described scenes of panic and confusion as youngsters scrambled to escape the blaze.

Samuel Rapp, who had been at a restaurant nearby, said he saw "a lot of people at the entry trying to escape. And it was full and people [were] walking on everybody trying to escape."

He saw many "screaming, trying to escape" and others "on the floor".

Mr Rapp told Sky News he saw "a lot of people" whose hair had been burned off, and others "without clothes".

'We had three chances to get in': Teenager on Swiss fire horror

Video from inside the bar shows a fire breaking out and rapidly engulfing the ceiling, spreading panic through the partygoers, who rushed towards the stairs to escape.

Later footage from outside showed lines of ambulances queuing and helicopters arriving to airlift victims to nearby hospitals.

Community grieves after deadly Swiss fire

Leon Christe, who works in the resort, told Sky News he was one of the first to the scene and tried to help by administering first aid.

He said: "It was very chaotic and very sad. Everywhere, there were young people who needed help, some had their clothes burned off.

"It was very traumatising."

Another witness speaking to BFMTV described people smashing windows to escape the blaze, some severely injured, and panicked parents rushing to the scene to find out what had happened to their children.

The young man said he saw about 20 people scrambling to get out of the smoke and flames, and likened what he saw from across the street to scenes from a horror movie.

Image: Mourners gathered to lay flowers. Pic: AP

Vigil for victims, with many details unknown

On New Year's Day, people gathered to lay flowers and light candles at a memorial near the site.

Little information has yet emerged about the victims, and hospitals and emergency services are reeling from the accident.

Some are being treated in the local hospital, while others were taken further afield, including to the University Hospital of Lausanne, which treats severe burns, and to France.

France said at least three nationals had been injured and eight were still missing.

Italian officials said 13 citizens had been injured, and another six are still unaccounted for, although details are still unfolding.