Russian First Deputy Prime Minister Igor Shuvalov says Ukraine cannot be in the European Union and the Russian-led Customs Union at the same time.
Shuvalov made the remark after talks between Russian Prime Minister Dmitry Medvedev and Ukrainian Prime Minister Mykola Azarov in Moscow.
Earlier, Medvedev urged Azarov to discuss "the legal, organizational, and economic consequences" if Kyiv signs a free-trade agreement with the European Union.
Moscow wants Kyiv to abandon a possible association agreement with the EU and is pushing it to join the Customs Union of Russia, Belarus, and Kazakhstan instead.
Russian customs officials placed extra checks on Ukrainian imports for several days this month, causing massive backups at the border.
It was seen as Moscow pressuring Kyiv over the Customs Union issue.
Ukraine hopes to sign a much-delayed free-trade and political association agreement with the European Union at a summit in Vilnius in November.
Based on reporting by ITAR-TASS and Interfax