Where did this happen?
At a bar called Le Constellation in the luxury Swiss ski resort of Crans-Montana, in the canton of Valais in southeastern Switzerland.
New Year's Eve celebrations were in full swing at the bar at the time of the incident.
How many people were in the bar?
A police spokesman earlier said some 100 people had gathered there for New Year celebrations.
The bar has a capacity of 300 people inside and 40 on its terrace, according to the Crans-Montana website.
"The party was in full swing... music and champagne flowing freely," a resident who said they lived nearby told 24 heures, a Lausanne newspaper.
When did this happen?
Authorities say the fire was first reported around 1.30am local time.
How quick was the emergency response?
Fire units were mobilised from across the region and "a large contingent of police, firefighters, and rescue workers immediately went to the scene to assist the numerous victims," police said in a statement.
Ten helicopters and 40 ambulances were sent to the scene, officials said.
How many people have been killed or injured?
According to the latest information from a police conference on Thursday morning, around 40 people are believed to have died. Officials are trying to get the victims' bodies to their families.
A further 100 were injured in the explosion with several sustaining "severe burns", officials said.
Local hospitals' intensive care units are already overwhelmed, so some patients are having to be taken to other cantons for urgent treatment.
Speaking at the press conference, Frédéric Gisler, police commander for canton Valais said that the injured had been taken to various hospitals, including Sion, Lausanne, Geneva and Zurich.
"As I speak to you, all the injured are being treated," he said.
What do we know about the victims?
As of midday on Thursday, few details have been released about the victims
Gisler said it was likely that the several hundred people affected were of different nationalities given the resort's popularity with tourists, but no further details were available so far.
Initial reports from the French foreign ministry said that at least two French citizens were among the injured.
Why do we not know the exact figures yet?
The investigations are still ongoing and officials have said it will take time to find out who has been killed and injured.
Support from neighbouring cantons would be needed to identify victims, prosecutor Béatrice Pilloud said at the press conference.
Could this be a terrorist attack?
No, police have ruled out the possibility of a terrorist attack.
So was it a fire or an explosion?
The incident was originally referred to as an "explosion of unknown origin" by police spokesperson Gaëtan Lathion.
But at the press conference, prosecutor Béatrice Pilloud said the incident was being considered a fire not an "attack".
So what do we know about the cause?
The investigations are still ongoing, but Swiss media suggested that the fire may have started when pyrotechnics were used during a concert. However, police said the cause was still unknown.
Two young French women, Emma and Albane, told French broadcaster BFMTV that they had been able to escape the "panic" in the bar shortly after the fire broke out.
They said "birthday candles" placed on champagne bottles had gotten too close to the ceiling.
"Seconds later, the entire ceiling was burning," one of them told the broadcaster, estimating that there were around 200 people in the venue at the time, mainly aged between 15 and 20.
It was also too soon to say whether the bar complied with safety regulations, officials said.
The Zurich Forensic Science Institute are helping the Valais authorities with the investigation.
How did witnesses describe it?
A tourist from New York filmed bright orange flames pouring from the bar, and told AFP he saw people running and screaming in the dark.
"With the fireworks, we didn't understand what was going on at first. Then we saw the smoke. It's terrible, a lot of young people go to that bar."
What's happening at the site?
The operation is still ongoing and the Valais government has declared a state of emergency in order to speed up the mobilisation of resources.
Crans-Montana has been closed off and police have imposed a no-fly zone over the area.
Ambulances were still parked outside the bar hours later, and broken windows could be seen. Local media described a "smell of burning still in the air".
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