Russia has demanded changes to a landmark political and economic pact between the EU and Ukraine in order to meet Moscow's concerns, according to a letter seen by AFP on Friday.
The European and Ukrainian parliaments on Tuesday simultaneously ratified the association agreement, the rejection of which last year triggered the political crisis in the former Soviet state.
The EU has firmly ruled out making any changes to the agreement, although a week ago it delayed the implementation of the pact's trade elements from this November until December 2015 under pressure from Moscow.
But in a letter seen by AFP from Russian Economy Minister Alexey Ulyukaev to the EU's Trade Commissioner Karel De Gucht, Russia says further talks on the pact should "clearly fix a mandate" for "the proposal for amendments to the Association Agreement".
The letter said this should include the "EU allowing for legally binding formulas to remove the concerns of the Russian side."
A European Union official on Friday confirmed the existence of the letter, which was dated Monday, adding that a reply was currently in preparation.
Moscow has been pushing for major changes to the deal, worried that the free flow of goods from the EU to Ukraine would prove highly disruptive to the Russian economy, which still has deep ties to its former Soviet republic.
In an effort to ease those concerns, the European Commission launched trilateral talks with Russia and Ukraine, that began at an expert level in July and culminated with last week's agreed delay.
German Chancellor Angela Merkel hinted at a softening in the EU's position in August when she said she wanted to "find a way that doesn't harm Russia."
But the Commission has repeatedly ruled out Russian demands to amend the agreement, the very deal whose last-minute refusal last year by Ukraine's then president Viktor Yanukovych plunged the country into chaos.
"The text is now finalised and ratified and the only way to change it would be the EU or Ukraine to re-open negotiations," the European source told AFP.
"We believe the concerns can be addressed within the existing agreement," the official added.
New Ukrainian President Petro Poroshenko drew sharp rebukes from even his staunchest supporters for bowing to Russian pressure and postponing the implementation of the EU trade deal.
But many observers agreed the delay could help avert the threat of a furious Russia unleashing crippling trade restrictions on Ukraine that would sink its economy even further.