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Ukraine's reliance on Russian gas is a particularly thorny issue in bilateral tiesUkraine's President Viktor Yanukovych has said he cancelled a visit to Moscow in December because his country was not yet ready to join Russia's customs union with Belarus and Kazakhstan.
In a newspaper interview he said Ukraine was trying to meet the legal requirements of the union without harming other international accords.
But Mr Yanukovych did not commit Ukraine to joining the union.
Ukrainewants closer ties with the EU, but also wants cheaper Russian gas.
Ukraine's gas imports from Russia have long been a thorny issue. In January 2009 a row over the gas price led to Russia halting gas supplies to Ukraine during a bitter winter - and that caused temporary gas shortages in much of Europe.
"Ukraine should consider the possibility of adapting its national legislation to those provisions of the customs union that would not contradict our international obligations," Mr Yanukovych told the Ukrainian edition of Russia's Komsomolskaya Pravda daily.
"Experts from both sides [Ukraine and Russia] are currently working on this issue," he said, adding that their work had not been finished in time for his planned visit to Moscow on 18 December, and that was why he cancelled it.
Referendum
Russiahas told Ukraine that it can get gas at the same lower price as Belarus if it joins the union, according to a Russian foreign ministry official quoted by the Interfax news agency.
But Alexander Gorban said Ukraine could not both aspire to join the EU and join the Russian-led customs union under conditions advantageous to Kiev. "It does not happen that way. One cannot be just a little pregnant," he said.
Mr Yanukovych has said Ukraine would have to change its constitution in order to join the customs union, and to do so might require a referendum.
But in Friday's interview he also noted that the other ex-Soviet countries in the union were a "huge market for Ukrainian products". He put Ukraine's annual trade turnover with them at more than $60bn (£38bn).
In December, Russian President Vladimir Putin said there were "economic problems" holding up Ukraine's accession to the customs union, including a disagreement over import quotas.
He insisted that Russia's long-term gas contract with Ukraine was not in dispute now, but he said Ukraine had made a "strategic mistake" by refusing to lease its gas pipeline network to Russia's Gazprom and other European operators.