President Viktor Yushchenko yesterday defended himself against Moscow's charges of anti-Russian policies and invited Russian leader Dmitry Medvedev to bilateral talks.
Mr Medvedev's comments, in which he said he wanted to see a new leader in Ukraine who was easier to deal with, marked a downturn in ties between Moscow and Kiev which have slumped since the 2004 pro-Western "Orange Revolution" swept Mr Yushchenko to power.
The Russian leader's open letter was seen by analysts as a warning shot to Ukrainian candidates in a presidential election next year, which Mr Yushchenko is unlikely to win.
"I will be frank - I am very disappointed with its unfriendly character," Mr Yushchenko said of Mr Medvedev's comments in a statement addressed to the Ukrainian people.
"I cannot disagree that there are serious problems in relations between our countries, but it is surprising that the Russian President completely shrugs off Russia's responsibility for this," he said.
Mr Yushchenko has irritated Moscow with his bid for Ukraine's Nato membership and insistence that Russia vacates its Black Sea Fleet based in the Crimea, a Ukrainian peninsula populated by mainly ethnic Russians.
Rows over subsidised gas from Russia and energy bills unpaid by Kiev have also led to Moscow turning the gas taps off to Ukraine in January, leaving millions of Europeans in the cold. Mr Yushchenko also backed Georgia in its brief war for its breakaway regions with Russia a year ago and was angry Moscow used warships from the Black Sea Fleet in its operations there.
Moscow in turn was furious to find Ukraine had been supplying arms to Georgia, but Mr Yushchenko said yesterday the arms sales were completely legal as no embargo had been placed on sales to Georgia by any international organisation.
Ukrainian analysts noted Mr Yushchenko's calm tone in contrast to Mr Medvedev's harsh criticism and said it could serve to show Mr Yushchenko to be more reasonable on the one hand and try to win favour with Ukraine's Russian-speakers ahead of the election.
"The tone and general mood is optimal - with no hysteria or passion, but with the message that instead of raising the temperature of the tensions, problems must be solved," said Volodymyr Fesenko, director of the Penta think-tank.
With just four per cent support however, Mr Yushchenko is likely to lose the January 17 poll and Prime Minister Yulia Tymoshenko and opposition leader Viktor Yanukovich are seen to want better relations with Moscow. The latest diplomatic spat follows Kiev's expulsion of a Russian diplomat last month, accusations that Russia tried to transport rockets without agreement out of its base and the delay over the appointment of a new Russian ambassador to Kiev.
The Kremlin said later yesterday Mr Medvedev had appointed Mikhail Zurabov as the new ambassador, though he is yet to send the envoy to Kiev and Mr Medvedev has said the timing would depend on how this diplomatic spat develops.