All 20 people on board a vintage plane have been killed when the Junkers Ju-52 crashed in the Alps of central Switzerland.

“The aeroplane crashed almost vertically and at high speed into the ground,” Daniel Knecht from the Swiss office of security investigations, which investigates such accidents, said at a Sunday press conference in Flims.

The 79-year-old aircraft, operated by tourist company Ju-Air, crashed on the western flank of Piz Segnas mountain at a height of 2540 metres on Saturday.

The dead, 17 Swiss and three Austrians, were aged between 42 and 84.

Multiple helicopters had been deployed to assist rescue and salvage crews.

News of the crash had come hours after another small plane came down around 10 kilometres south of Lucerne in central Switzerland, killing four members of a family.

Investigators are still in the dark about the cause of the Piz Segnas accident, though Kurt Waldmeier, the co-founder of Ju-Air, suggested it could be linked to the ongoing heatwave in Europe.

“High temperatures can affect the performance of an aircraft,” Knecht said, adding that a pilot may require more experience to handle such conditions.

Waldmeier said the 62-year-old pilot had more than three decades of flight experience with Swissair Airbus and in the air force. The co-pilot was also very experienced, he added.

-AAP