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MOSCOW, Dec 5 (Reuters) - Russia has received an official refusal from the U.S. government to provide guarantees for Ukraine's liabilities, Russian news agencies reported on Saturday, citing Russia's Finance Ministry.

Russia and Ukraine are at loggerheads over a $3 billion Ukrainian Eurobond held by Russia and maturing on Dec. 20.

Russia has offered to restructure the debt in equal instalments over the next three years if the West provides guarantees. But Ukraine has included it among the sovereign and sovereign-guaranteed bonds to be restructured in a deal with a group of its largest creditors.

"This week we received an official refusal from the U.S. government to provide guarantees on Ukraine's liabilities. In this regard, there is nothing left but for us to file a legal case against Ukraine if the debtor does not fulfil its liabilities fully on Dec. 20," RIA news agency cited the Finance Ministry as saying in a statement.

The ministry added that Russia had offered Ukraine better terms than those the International Monetary Fund had asked for.

Ukraine is restructuring its debts in order to plug a $15 billion funding gap under an IMF-led $40 billion bailout programme.

The Eurobond is especially contentious as it was issued by the government of former president Viktor Yanukovich in late 2013 as public protests took off against his swing away from association with the European Union towards deeper ties with Russia.

Unrest and bloodshed eventually forced Yanukovich to flee. (Reporting by Alexander Winning; Editing by Toby Chopra)

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