Everything we know about the deadly New Year's Eve explosion at Swiss ski resort as 'several dozen' believed to be killed

An explosion at a ski resort bar in Crans-Montana, Switzerland, has left 'dozens' dead and many seriously injured.

The fire occurred during a New Year's Eve celebration at a venue called La Constellation, which was attended by over 100 partygoers, who were seen trying to escape the blaze.

Footage shared on social media - which remains unverified - appears to show a fire burning inside the bar, while other images show a fire engine and ambulances outside the bar.

Updates from officials are continuing to come in.

Here's everything we know so far.

At least 100 people were in the bar when the incident occurred (MAXIME SCHMID / AFP via Getty Images)

Where and when did the explosion happen?

The tragedy happened during the early hours of Thursday (1 January) during a New Year's Eve celebration at Le Constellation, a bar located in the Alpine ski resort of Crans-Montana.

The resort is in the Valais region in the middle of the Swiss Alps - a popular destination with European and British tourists.

Crans-Montana is around 40km north of the Matterhorn and around two hours from Bern, the Swiss capital.

What happened?

At around 1.30am local time (12.30am UK time), a fire broke out at the bar. The cause of the blast is currently unknown but officials have ruled out it being an attack.

A witness told French broadcaster BFMTV that people smashed windows to escape and likened what he witnessed to that of a horror movie.

Reports claim different things were to blame for the fire, with Blick, a Swiss news outlet, saying it may have been caused by pyrotechnics from a concert.

Elsewhere, Italian Foreign Minister Antonio Tajani told Italy's Sky TG24 that fireworks could have been behind the explosion, while two women who claim to have escaped the fire told BFMTV that birthday candles placed by staff on champagne bottles had prompted the incident.

One of the women told the outlet: "One of them was brought too close to the ceiling, which caught fire. In a few dozen seconds, the whole ceiling was on fire. Everything was made of wood."

Gaetan Lathion, a police spokesperson in Wallis canton, in southwestern Switzerland, told AFP: "More than a hundred people were in the building, and we are seeing many injured and many dead."

It has been confirmed that Swiss authorities are not treating the incident as an attack. Authorities have opened a full investigation into the fire.

Beatrice Pilloud, a prosecutor, said at a press conference: "At the moment we are considering this a fire and we are not considering the ‍possibility of an attack."

Police have secured the scene of the explosion (MAXIME SCHMID / AFP via Getty Images)

How many were injured or killed?

Speaking at a news conference on Thursday morning, Swiss police said 'dozens' of people are believed to have been killed in the fire at La Constellation.

Around 100 other people have been hurt, with many said to be seriously injured.

"This evening should have been a moment of celebration and coming together, but it turned into a nightmare," said Mathias Renard, head of the regional government.

He also said that the intensive care unit at the local hospital was full and patients are being transferred to other facilities across the country. 40 ambulances and 10 helicopters were mobilized once the alarm was raised, Renard added.

An exact number of fatalities is yet to be confirmed by authorities.

The Italian foreign ministry claimed Swiss authorities have attributed at least 40 deaths to the incident, adding that several victims could not be identified due to severe burns.