Strasbourg: EU foreign affairs head Federica Mogherini told AFP on Wednesday that a high-stakes Ukraine peace summit in Minsk would be a turning point in the conflict.
Mogherini also urged Moscow to take the chance to become a "responsible actor" and help end the 10-month war between Kiev and pro-Russian separatists.
"It is something (that) is going to be a turning point for good or for bad... if tonight in Minsk we manage to reach an agreement," Mogherini said in an interview.
"If we do not, then all the options, all the developments that one can imagine for Ukraine itself, but also for Europe, for relations between Russia and the EU and the rest of the world, definitely, one can say (they would be) at the least worrying."
Her comments came hours before the leaders of Ukraine, Russia, France and Germany were to hold peace talks in the Belarussian capital aimed at ending the crisis.
She conceded that even if they thrashed out a peace deal, there would still be challenges ahead.
"If in Minsk we manage to get an agreement, then obviously we still have a difficult time in front of us," Mogherini said in the interview, held to mark her first 100 days in office this week.Mogherini would not comment directly on the question of whether to arm Kiev against the rebels, an option US President Barack Obama has said he has not yet decided on.
Germany leads many countries in the European Union who fear the weapons option will only stoke a conflict with a nuclear-armed and increasingly assertive Russia. The EU has imposed sanctions on Russia, accusing it of supporting the rebels, but there is little appetite among its 28 members for sending weapons to Kiev.
Instead Mogherini, a former Italian foreign minister accused by some eastern European states of being soft on Moscow, focused on persuading Russia to cooperate.
"Russia has to get out of this idea that it has at the moment that relations with the EU are confrontational," she said.
"There is a chance (in Minsk) for Russia to use its influence with the separatists to restore peace and also, hopefully, to return to respect of international law."
"It is a chance for Russia to show that it is a responsible actor. The starting point is stopping the conflict in east Ukraine."Asked about sharp divisions over the conflict within the EU, particularly with a new Kremlin-friendly Greek government, Mogherini said that was no surprise but that all members recognised the absolute importance of unity.
"It is very naive to say we are all united and think the same way," Mogherini said.
"What surprises Putin is our unity and I can guarantee we are going to keep that unity," she added, citing a recent decision to increase targeted sanctions on Russian and Ukrainian figures pending the outcome of the Minsk talks.
Mogherini also urged Russia not to perceive the EU`s so-called "Eastern Partnership" plan for former Soviet states as a threat.
The Kremlin has watched with alarm as both the EU and NATO have expanded since the break-up of the Soviet Union towards Russia`s borders.
"We have to be clear that Ukraine, Moldova and Georgia have the sovereign decision about their future, and their present," she said.
"It`s not a question of building a new wall, of trying to create a space between us. The objective has to be to create a space for cooperation.