Pro-Russian separatists have decided to proceed with a referendum on independence as planned, despite a call from Russia's President Vladimir Putin to postpone it.

Denis Pushilin, the leader of the self-declared Donetsk People's Republic, said on Thursday that the coordinating committee of the rebels had decided in a meeting that the vote would be held on Sunday as planned.

"We would have lost the trust of the simple people [if we had not gone ahead]," Pushilin said.

In a similar move, the rebel group of neighbouring Luhansk region, the Army of the Southeast, told Russia's RIA news agency that they also rejected Putin's appeal to delay the referendum scheduled for May 11.

A spokeswoman for the separatists in Slovyansk simultaneously confirmed to AFP news agency that the vote would be held on the same day in the city too.

Al Jazeera’s Jonah Hull, reporting from Donetsk, said the reality was that the separatists formed a minority in the region, and there would be no way to measure the fairness of the voting.

"There are no monitors, no formal structures and no ways of measuring the veracity of the results when they come in," he said.

Against this backdrop, Catherine Ashton, EU foreign policy chief, said on Thursday a decision to go ahead with the vote that had "no legitimacy" could "only further worsen" the situation in Eastern Ukraine.

Ukraine's interim government and its allies blame Russia for fomenting the crisis in the country's east, but Russia denies involvement.

Ukraine's security service claimed on Wednesday to have obtained "conclusive evidence proving that the Russian Federation is preparing and coordinating activities aimed at holding in eastern Ukraine a so-called 'referendum' on creation of the Donetsk People's Republic".

The service published a telephone conversation online that allegedly took place on May 5 between a leader of the Russian National Unity movement and one of the leaders of the pro-Russian Orthodox Donbas organisation.

For his part, Putin said on Thursday that the developments in Ukraine show where "irresponsible politics" led, blaming the government in Kiev for unrest in the east.

Ukrainian officials earlier on Thursday rejected Putin's call for them to halt a military operation against the rebels, who have seized more than a dozen towns.