The airspace above the crash site was closed by the Federal Office for Civil Aviation and access to popular hiking trails in the surrounding area was blocked.
Photos released by Graubuenden canton (state) police showed the crumpled wreckage of the plane lying on the mountain, with only the upside-down tail more or less intact.
Police said that the aircraft had burned and officials haven't yet been able to ascertain the exact number and identity of those who died. They also expect the investigation into the cause of the crash to be "relatively complex".More news: Apple Becomes First Company to Reach $1 Trillion Worth
JU-Air said on its website that it was "deeply saddened" and its "thoughts were with the passengers, the crew and families and friends of the victims". Authorities were informed of the crash at 5 p.m. Saturday, 50 minutes after the aircraft had taken off from Locarno's Magadino airfield.
The plane was built in 1939 and can carry up to 17 passengers and three crew members.
On its website, JU-Air mentions one past accident, in 1987, at the Koblenz airport in Germany in which nobody was hurt.More news: Jordan army clashes with Islamic State militants fleeing Syrian forces
The aircraft have three engines, one on the nose and one on each wing.
Police have confirmed the crash, saying in a tweet that it occurred on the western side of the mountain Piz Segna (top picture) at about 2,540 meters (8,333 feet) above sea level on Saturday. There was no immediate word on casualties.More news: Apple's Market Cap Hits $1 Trillion