"The vote will happen on May 11," the leader of the self-proclaimed People's Republic of Donetsk, Denis Pushilin, declared to reporters Thursday.

There were fears that Ukraine could still erupt in fresh violence on Friday when both it and Russia celebrate the Soviet victory in World War II.

There were some reports that Putin could make a triumphant entry into Crimea, which was annexed by Russia from Ukraine in March.

German Chancellor Angela Merkel voiced her concern to Putin about this possibility, her Foreign Minister Frank-Walter Steinmeier said Thursday. "He left the answer open about whether he would personally attend".

The Cold War-style tension was ratcheted up another notch on Thursday as Russia conducted military drills, including test-firing ballistic missiles, and said Ukraine would now have to prepay for its crucial gas imports.

A display of tanks, missile systems and fighter jets on Red Square Friday will help mark the WWII victory over the Nazis.

Pushilin said Thursday, to applause from members of the "republic's" ruling council, that "the people's desire to hold the referendum is becoming even greater."

On Sunday, people in the restive eastern region will be called to answer one simple question: "Do you support the declaration of independence by the Donetsk People's Republic?"

Separatists in the other main rebel-held towns of Slavyansk and Lugansk also declared they would hold a plebiscite.

The European Union, whose foreign ministers meet on Monday to consider further sanctions against Russia, said the referendums "could have no democratic legitimacy and would only further worsen the situation".

The referendum move dashed hopes of diffusing the crisis after Putin on Wednesday made a surprise call to the rebels to postpone their plebiscites.

In a stunning about-face, the Kremlin strongman also backed a presidential election planned by Kiev's interim leaders on May 25 that Moscow had only recently described as "absurd".

Putin had previously said that Kiev must cease its military operations in the east in return for Moscow's backing of the May 25 election.

But on Thursday, Kiev vowed to press forward with what it calls an "anti-terrorist" operation against insurgents holding a dozen or so towns and cities in the east.

"The counterterrorist operation will go on regardless of any decisions by any subversive or terrorist groups in the Donetsk region," Andriy Parubiy, secretary of Ukraine's national security and defense council, told reporters.

BA/BA