British foreign minister William Hague said in Kiev on Wednesday that for a second round of international talks on Ukraine to be worthwhile Russia must accept Ukrainian elections going ahead on May 25 and cease interference disrupting them.
Hague, speaking after meetings with Ukrainian politicians, also said parties to any follow-up to Geneva talks, which failed to produce any solution to conflict in Ukraine, must be fully committed to implementing its outcome.
"Russia didn't take a single step to implement what was agreed in Geneva," he said, referring specifically to Russian undertakings to use its influence to dampen separatist activity.
"There must be ... a complete acceptance that Ukraine is entitled to have elections on May 25 and that these should go ahead without external disruption," the foreign secretary said.
Russia challenges the legitimacy of the Ukrainian elections, arguing that the authorities calling them are themselves illegal, having emerged from the overthrow of pro-Russian president Viktor Yanukovich. Kiev is currently not in a position to conduct the polls in large parts of the east.
"Failure to hold these elections would be very serious," Hague said. "Once postponed, who knows when they would be held?"
On Tuesday, Russian Foreign Minister Sergei Lavrov said it would be "unusual" to hold an election in Ukraine while the army was being deployed. Pro-Russian separatists in the eastern regions of Donetsk and Luhansk, centre of a violent uprising, are holding their own referendum on independence on Sunday.
The frontrunner in polls for the presidential election, Petro Poroshenko, said on Wednesday he was willing to hold a referendum on constitutional change but not while conflict continued. He urged Western powers to impose further sanctions on Moscow if it supported the separatist vote on Sunday.
Hague said Russia was deeply involved in the insurgency in the east, where Ukrainian troops are trying to retake control of a string of cities now under rebel forces' control.
"It is clear that leading elements in these forces ... are not simply pro-Russian, but parts of them have been Russian," he said.
Russian denies accusations it is co-ordinating rebellion in eastern Ukraine and deploying its special forces there to lead operations.