UPDATE 1-Putin says will be hard to work with Ukraine's new leaders
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SHANGHAI May 21 (Reuters) - President Vladimir Putin said on Wednesday it will be hard for Russia to build relations with Ukrainian leaders who come to power while fighting persists in the country's east.
In the latest in a string of comments from Moscow casting doubt on the legitimacy of a presidential election scheduled for Sunday, Putin also said it would be more logical for Ukraine to hold the vote after a referendum on a new constitution.
Russian officials have suggested the vote will not be legitimate without an end to hostilities between government forces and pro-Moscow separatists and implementation of a road map to resolve the crisis in the former Soviet republic.
"It will be very hard for us to build relations with people who come to power against the backdrop of a continuing punitive operation in southern and southeastern Ukraine," Putin said, referring to the anti-separatist operation by government forces.
In Bucharest, U.S. Vice President Joe Biden warned that undermining the May 25 election would bring greater costs for Russia - already subject to Western sanctions since its annexation of Crimea.
Russia wants Ukraine to adopt a constitution that would give a powerful measure of autonomy to its regions, which Moscow says is needed to guarantee the rights of Russian-speakers in the east.
"In my view it would be far cleaner, from a legal point of view ... to first hold a referendum on all the basic issues, a constitutional referendum ... and then elect a president and parliament on the basis of this constitution," Putin said.
"...What is important is not the election itself but establishing relations with all regions of Ukraine, so that people feel like citizens no matter where they live." (Reporting by Alexei Anishchuk; Writing by Steve Gutterman; Editing by Alissa de Carbonnel, John Stonestreet)