Dozens of people are feared to have been killed and around 100 injured after an explosion tore through a crowded bar during a New Year’s Eve party in the upscale ski resort of Crans-Swiss police said on Thursday.

The Italian foreign ministry said information from Swiss police indicated about 40 deaths, but police would not be more specific than "tens" at a press conference on the incident.

The fire broke out at 1.30 am (0030 GMT) in a bar called "Le Constellation" in the resort in southwestern Switzerland.

Some of the victims are from other countries, said Stephane Ganzer, head of security for the Valais canton. Authorities said 10 helicopters and 40 ambulances had been mobilised.

Police had earlier said many people were being treated for burns. Most of those injured were in a serious condition, said Mathias Reynard, head of the cantonal government. Valais hospital intensive care unit was full and patients being transferred elsewhere, he said.

The area has been completely closed off, and a no-fly zone has been imposed over Crans-Montana, police said in a statement, adding that the cause of the blast remained unclear.

The explosion was being considered a fire and not an attack, prosecutor Béatrice Pilloud said later, adding that authorities were trying to get the bodies of the victims to their families.