Twenty people were killed when a vintage plane, carrying tourists on a panoramic flight, crashed into a Swiss mountain, police said Sunday.

Three crew and 17 passengers were on a sightseeing tour on Saturday when the plane "fell to the ground like a stone," according to Swiss newspaper 20 Minutes.

"The police have the sad certainty that the 20 people aboard perished," police spokeswoman Anita Senti told a news conference.

Images show the plane on the ground and emergency services, including a helicopter on the scene.

The plane crashed 2,540 meters above sea level into the Piz Segnas mountain in east Switzerland on Saturday.

The cause of the crash is unknown, and Swiss officials said they were not aware of any distress call from the plane.

Daniel Knecht of the Swiss Transport Safety Investigation Board said the plane hit the ground near-vertically and at high speed, although officials expect the investigation to be "relatively complex."

Speaking in the Alpine resort of Flims, Mr Knecht said the vintage plane presumably did not have the flight data recording system more modern aircraft have.

Mr Knecht also said officials ruled out foul play and collision with an obstacle, or that the plane lost parts or broke up before the crash, as a reason for the incident.

Operator JU-Air said it was saddened by the news, and that it had set up a helpline for relatives. It suspended all flights.

"The JU-Air team is deeply saddened and thinks of the passengers, the crew and families and friends of the victims," the company wrote on their website.

The Junker JU52 HB-HOT aircraft was a World War Two plane built in Germany in 1939. At least 5,000 Ju-52 planes were manufactured between 1932 and 1952.