The Abkhaz parliament ruled out forming a federation with Georgia, a proposal offered by Tbilisi and backed by Russian mediators.
The parliament said Wednesday that Abkhazia is a sovereign state, Itar-Tass said.
A high-ranking Russian Foreign Ministry official told Interfax the separatists' decision was "counter-productive" and said Moscow would respond "in the harshest way".
"This is at least a short-sighted step which contradicts the long term interests of the Abkhaz people", Interfax quoted the official as saying.
The official did not say what the measures would be.
Abkhaz parliament speaker Sokrat Dzhindzholia was quoted by Itar-Tass as saying a federation with Georgia was "unacceptable".
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