Russian investigators have opened a murder case after two fishermen were rescued following three months lost in a remote far east forest amid fears the pair could have eaten a companion to stay alive, officials say.

Four men disappeared in August on a river-fishing expedition to the vast Yakutia region in the Russian Far East, one of the most remote and inhospitable places in the world.

Rescuers finally found two of the men this month by the Sutam River some 250 kilometres from the nearest town of Neryungri in the south of Yakutia, but without two companions.

The men claimed their group had split up and said the others were likely still alive, as they were used to living in the open.

But a murder probe was opened after a team of top investigators from the regional capital Yakutsk found fragments of a human corpse close to the place where the pair was found.