A sealed off building with emergency service workers on the perimeter.

The area has been completely closed off, and a no-fly zone has been imposed over Crans-Montana, according to police. (Maxime Schmid/AFP)

Dozens of people are feared to have been killed and around 100 others injured after an explosion tore through a crowded bar during a New Year’s Eve party in the luxury Swiss ski resort town of Crans-Montana, local authorities have said.

The majority of the injured are seriously hurt, and Valais Hospital’s Intensive Care Unit is full, with patients being transferred elsewhere.

Police have said some of the victims are from other countries, but their nationalities are yet to be confirmed.

The “fire of undetermined origin” began at 1:30am, local time, on Thursday in a bar called Le Constellation in south-western Switzerland as revellers rang in the New Year, according to police.

The bar has a capacity of 300 people and another 40 people on its terrace, according to the Crans-Montana website.

The spokesperson said some 100 people had gathered there for New Year’s Eve celebrations.

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.

A sealed off building with emergency service workers on the perimeter.

Rescuers at the site of an explosion at a bar in Crans-Montana, Switzerland. (Maxime Schmid/AFP)

Police said a large contingent of emergency services were mobilised including 10 helicopters and 40 ambulances.

The area has been completely closed off, and a no-fly zone has been imposed over Crans-Montana, police said.

Swiss media suggested that the fire may have started when pyrotechnics were used during a concert, but police said the cause was unknown.

While the investigation is ongoing, authorities have stressed that it is being treated as a fire and not an attack.

Police spokesperson Gaetan Lathion told AFP earlier there had first been an “explosion of unknown origin”.