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Passers-by were too late to stop the attack by the black squirrels in a village in the far east, which reportedly lasted about a minute.
They are said to have scampered off at the sight of humans, some carrying pieces of flesh.
A pine cone shortage may have led the squirrels to seek other food sources, although scientists are skeptical.
The attack was reported in parkland in the centre of Lazo, a village in the Maritime Territory, and was witnessed by three local people.
A "big" stray dog was nosing about the trees and barking at squirrels hiding in branches overhead when a number of them suddenly descended and attacked, reports say.
"They literally gutted the dog," local journalist Anastasia Trubitsina told Komsomolskaya Pravda newspaper.
"When they saw the men, they scattered in different directions, taking pieces of their kill away with them."
Mikhail Tiyunov, a scientist in the region, said it was the first he had ever heard of such an attack.
While squirrels without sources of protein might attack birds' nests, he said, the idea of them chewing a dog to death was "absurd."
"If it really happened, things must be pretty bad in our forests," he added.
Komosmolskaya Pravda notes that in a previous incident this autumn chipmunks terrorized cats in a part of the territory.
A Lazo man who called himself only Mikhalich said there had been "no pine cones at all" in the local forests this year.
"The little beasts are agitated because they have nothing to eat," he added.