Belarusian president Alexander Lukashenko has lashed out at Russia over its withdrawal of gas subsidies, saying relations between the two countries could be severely damaged.
Russia's government announced on Monday that its giant state-controlled gas firm Gazprom will stop selling cheap gas to Belarus from the start of next year.
Prime Minister Mikhail Kasyanov has signed a resolution to cancel a previous order, which sold the gas to Belarus at a cost three times lower than the gas it sells to Germany.
But Belarus, which is almost totally dependent on Russian gas for its energy supplies, could lose up to $700m a year, according to Russia's business newspaper, Kommersant.
"Our two peoples' striving for unity has suffered a most severe blow," Mr Lukashenko said in footage broadcast on Russian state television.
"If we overcome this, we can expect a normal future. But if we succumb to the blows of these scheming and powerful people, this will push back the unity of our two nations," he warned.
Analysts say the Kremlin has become increasingly unhappy with the policies of Mr Lukashenko.
Commentators have linked the decision to Mr Lukashenko's recent refusal to adopt the rouble as part of planned economic and political union between the two countries.
Belarus has maintained close ties with Moscow since the collapse of the Soviet Union.
The move by Gazprom is likely to have severe consequences for the republic already impoverished by its Socialist-style command economy and the iron-fist rule of Mr Lukashenko.
But Moscow may also suffer from the decision since Belarus is a key route for Russian gas being piped for
export to western Europe.