The European Union warned on Friday that it views any pressure by Russia against Ukraine as "unacceptable" as Kiev prepares to sign a landmark association accord with the EU.
"In the view of the EU, any economic threat from Russia directed against Ukraine and linked to country's possible signature of the association agreement with the EU is unacceptable," EU Trade spokesman John Clancy said.
He was responding to a question about comments by Russian President Vladimir Putin.
In the latest of a series of strained exchanges, Putin warned Kiev on Thursday that Russia and its partners in its Customs Union could not simply stand by if Ukraine opted for such an important political and trade tie-up with the EU.
"The countries of the Customs Union will have to think about protective measures, such a possibility exists," Putin said, according to Russian news agencies.
"Next week, our Ukrainian colleagues will come to Moscow to analyse on an inter-governmental level all the possible threats related to this situation," he added.
The EU-Ukraine accord has been repeatedly delayed by differences between Kiev and Brussels, most notably over the treatment of former premier Yulia Tymoshenko, but it now looks set to be signed at an EU Eastern Partnership summit in November in Vilnius.
It would open up EU markets for Ukrainian exports and mark a decisive turn towards the West, stoking concern in Moscow which considers Kiev to be firmly in its sphere of influence.
Last month, Putin met his Ukrainian counterpart Viktor Yanukovych in what analysts described as a last-ditch attempt to persuade Kiev against signing the EU deal.
With no progress apparently made, Russian customs then began subjecting all Ukrainian imports to rigorous checks in a move that a top Putin aide, Sergei Glazyev, publicly acknowledged was a warning to Kiev.
Ukraine said earlier this week that the increased customs checks had stopped and tensions with Moscow had eased.