French President Francois Hollande said on Monday that nothing must impede the upcoming presidential vote in Ukraine, adding that Russia must be persuaded against using violence.

"The presidential election of May 25 must be held and nothing must hinder it," Hollande told reporters at a joint press conference with visiting Japanese Prime Minister Shinzo Abe.

His comments came as European leaders, fearing an outbreak of full-scale civil war, are desperately seeking to force Ukraine and Russia to find a negotiated solution.

Hollande said the presidential ballot "must be held in all of Ukraine, which implies that a general agreement must be found so that the election and its result are accepted."

There have been heavy clashes around the eastern town of Slavyansk, the epicentre of the armed pro-Russian insurgency, which has seen the separatists holding more than two dozen towns.

Hollande said the aim was to "convince the Russian authorities and especially President (Vladimir) Putin that the only path possible is dialogue and therefore a de-escalation."

Abe said Moscow "continues to fan tensions" and underscored the need for a successful election in Ukraine.

Russia denies any hand in the violence in eastern and southern Ukraine.

French Foreign Minister Laurent Fabius earlier Monday slammed Russia for double standards, saying it was "contradictory" to oppose elections in Ukraine while urging Syrians to vote in elections due in June.

Moscow has criticised as "absurd" plans to hold the presidential election given the violence raging across Ukraine.

But this stance has raised warnings from the United States and Germany of tougher sanctions on the Russian economy.