Four people were killed Tuesday when a Mi-8 helicopter crashed in Russia's Far East Sakhalin peninsula, the RIA Novosti news agency reported.

"All those onboard have been killed," the news agency quoted an official as saying.

Earlier reports said three people died with just one body found.

"The Mi-8 was on a training mission, when it disappeared from radar screens. A number of local residents said they saw a flare," a police told Itar-Tass news agency.

The border guard helicopter fell down into a forest and caught fire, according to witnesses.

The cause of the accident remains unclear so far.