Russian investigators have opened a murder case amid fears two men who survived three months lost in a remote forest could have eaten a companion to stay alive.

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 found two of the men this month by the Sutam River, around 250 kilometres from the nearest town of Neryungri in the south of Yakutia.

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 investigators from the regional capital Yakutsk found fragments of a human corpse close to where the pair was found.

"Investigators carried out an examination of two areas," the Yakutia branch of Russia's Investigative Committee said in a statement.

"Fragments of a human corpse with signs of a violent death were discovered and removed.

"A criminal case into suspected murder has been opened."

Russia has no article in the criminal code for cannibalism, but the state RIA Novosti news agency said the initial theory was that the two men had eaten one companion.

It is not clear what happened to the fourth man.

The pair, aged 35 and 37, have denied any wrongdoing and said they had managed to survive as winter set in in a wooden hut by foraging for wild foods.

The Yakutia investigators said DNA and pathological testing had been ordered and they were working urgently to uncover what happened.

AFP