Police say a German World War II-era transport plane which crashed in the Swiss Alps, killing all 20 people on board, was seen nose-diving into the ground.

Key points: The deceased passengers were aged between 42 and 84

The deceased passengers were aged between 42 and 84 Police say there's no evidence of a distress call being made before the plane crashed

Police say there's no evidence of a distress call being made before the plane crashed The Ju-52 lacked black boxes that modern aircrafts have

The three-engined Junkers Ju 52 was carrying 17 passengers and three crew on a sightseeing flight when it crashed on Saturday afternoon on the west side of the Piz Segnas mountain in the canton of Grissons.

The aircraft had been returning from Locarno near Switzerland's southern border.

Its operator, JU-Air, has been running sightseeing tours for almost four decades using a squadron of German-made vintage aircraft.

The company JU-Air has been running sightseeing tours for almost four decades. ( AP: JU-AIR/Keystone )

Ju 52s were widely used by Hitler's Luftwaffe during World War II to transport troops and equipment. They were also used as civilian passenger planes, both before and after the war.

The plane which crashed was built in the late 1930s as a military aircraft and was later used to operate scenic and charter flights.

The passengers who died, aged between 42 and 84, were mostly from Switzerland.

The wreckage of the plane was in a basin at 2,450 metres above sea level surrounded on three sides by peaks, a Reuters witness said.

Police said they had not found any evidence of a distress call being made before it crashed.

"We can assume that the aircraft hit the ground near-vertically and at relatively high speed," Daniel Knecht of the Swiss Transportation Safety Investigation Board said at a news conference in Flims.

He and senior police official Andreas Tobler said the Ju 52 lacked "black boxes", the crash-resistant cockpit voice and data recorders that more modern aircraft have.

Officials expect the investigation of the cause to be, "relatively complex, because we have to compare various indications, information and evidence and evaluate them," Mr Knecht said.

There also are typically few radar recordings in mountainous areas such as the one where the crash site is located, he added.

'Older planes can be operated safely'

The plane crashed on the west side of the Piz Segnas mountain in the canton of Grissons. ( AP: Gaetan Bally/Keystone, file )

Officials could essentially rule out a collision with another aircraft or an obstacle such as a wire, Mr Knecht said.

There was also no indication of any "external influence", he said, indicating authorities did not suspect foul play.

The plane did not catch on fire before or after it hit the mountain, and investigators are yet to find any signs it lost parts or broke up in the air before the crash, Mr Knecht said.

He also dismissed the idea the plane's age was necessarily a problem.

"Older planes, if they are correctly maintained, can be operated safely," he said.

Nearly 5,000 Ju 52 planes were manufactured between 1932 and 1952.

The area around the crash site, which is popular with hikers and skiers and includes a glacier, was closed to the public.

Mr Knecht said authorities would probably need "a few days" to complete recovery work.

ABC/Wires