Ukrainian President Petro Poroshenko is threatening to abandon a peace plan with pro-Russian separatists after they held elections aimed at demonstrating their independence.
Sunday's election of leaders in the Donetsk and Luhansk regions of eastern Ukraine were backed by Russia, but condemned by Ukraine, the US and European powers.
Mr Poroshenko hit out at the "pseudo-elections" as a "brutal violation" of the 5 September truce deal, the Minsk Protocol.
"This farce at gunpoint has nothing in common with the expression of will," he said.
"It cannot be called an election by definition. Bandits, terrorists and interventionists can crown themselves deputies, prime ministers and ministers. They may even call themselves kings or emperors.
Pro-Russians celebrate at the ending of voting day in Donetsk
"Still, no matter what they put on their heads, they will remain occupants, criminals and militants."
The September accord, whose signatories included Russia, was intended to pave the way for an end to the seven-month conflict with a ceasefire and an offer of autonomy - but not independence - for the rebels.
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But Mr Poroshenko has said he and his security chiefs will "re-examine" Kiev's commitments to the ceasefire deal, including considering the "abolition" of the key law offering separatists autonomy within Ukraine's existing borders.
Alexander Zakharchenko, a former electrician turned insurgent leader, won the election in Donetsk, while Igor Plotnitsky, an ex-Soviet army officer, was elected in neighbouring Luhansk. They both won by large margins.
Mr Zakharchenko said: "Ukraine does not want peace, as it claims. Obviously it is playing a double game."
Russia, which risks an intensifying of tough EU and US economic sanctions, ignored Western appeals not to recognise the rebel vote, which demonstrated Ukraine's inability to control the east.
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"Those elected have received a mandate to resolve the practical issues of re-establishing normal life in the region," Russia's foreign ministry said in a statement.
Russia also blocked an attempt in the UN Security Council to criticise the elections.
The US followed Europe in slamming the rebel polls, which were held without recognised election observers.
The White House said: "These sham elections contravened Ukraine's constitution... and the most basic electoral norms," while the State Department warned Moscow that recognising the polls "would only serve to isolate it further".
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Gallery: Ukraine Crisis Escalates In Donetsk
As Ukraine's parliament ratifies an agreement for closer ties with the European Union, fighting continues in Donetsk
A pro-Russian rebel walks past burnt-out vehicles in the eastern industrial Ukrainian city
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