DONETSK, Ukraine -- Pro-Russian separatists in eastern Ukraine vowed Thursday to press ahead with a referendum on independence, defying Russian President Vladimir Putin's surprise call for Sunday's vote to be postponed.
Having captured government buildings across eastern Ukraine and vehemently denounced the interim government in Kiev as fascists, the leaders of the self-styled "Donetsk People's Republic" argued that they would lose credibility if they canceled the vote.
"Civil war has already begun," Denis Pushilin, a prominent leader of the group, told a packed news conference in Donetsk. "The referendum can put a stop to it and start a political process."
The decision to proceed with the vote could be seen as a rebuff to Mr. Putin, whose call Wednesday for a postponement struck a more conciliatory tone than his previous statements on Ukraine.
It remained unclear what a referendum might look like, who would participate, how fair it might be, or even in how many or which cities it would be held.
But the separatists clearly felt they had little choice but to press on: Canceling the vote would leave them without even a fig leaf of popular legitimacy and deflate their movement, perhaps fatally.
Ukraine's acting prime minister, Arseniy Yatsenyuk, said Thursday that any referendum would lack legitimacy. Ukraine has said questions about the country's future should be decided in nationwide presidential elections scheduled for May 25, not in any regional vote.
Feeding a sense that the letup in tensions had been fleeting, Mr. Putin on Thursday led major military exercises that simulated a response to a massive attack on Russian soil, defense officials in Moscow said.
Kremlin-backed television channels showed vast salvos being fired across Russia, including intercontinental ballistic missiles from submarines, cruise missiles from a Tupolev bomber and scores of Grad rockets raining down on a practice range.
Mr. Putin said the strikes were part of exercises that were planned in November to demonstrate the high readiness of the country's "strategic offensive and defensive forces."
The separatists called the referendum to decide whether the Donbas region of eastern Ukraine, the nation's industrial heartland of Donetsk and Luhansk, should declare independence.
But with little coordination or trust among separatist leaders in different cities across the region, it was far from clear what a putative new republic would look like.
There was also widespread skepticism about the separatists' ability to stage a referendum with even a minimum of credibility.
Boris Litvinov, a leader of the referendum effort, said about 3 million ballots have already been printed and 2.7 million of them distributed. The ballots ask voters whether they support the "independence of the Donetsk People's Republic."
But Mr. Litvinov said authorities in Kiev have denied the separatists access to voter rolls. Therefore, he said, the referendum would be an "open process" in which people would simply turn up at polling centers, show their passports, sign their names and cast their ballots.
After two days of mixed messages from Russia, Mr. Putin's real intentions about the referendum remained hard to read. Analysts in Moscow said he could be playing a double game, disassociating Russia from what is likely to be a deeply flawed contest while maintaining flexibility in how to respond.
The Pentagon and NATO continued on Thursday to dismiss a claim by Mr. Putin that Russian troops had been pulled back from the Ukrainian border. The disagreement sparked a Twitter spat between the Russian Foreign Ministry and NATO Secretar- General Anders Fogh Rasmussen, who said he had seen no sign of a withdrawal.
In the streets of Donetsk, confusion reigned Thursday about what the vote really means, and there appeared to be agreement on only one thing: People want peace and stability to return.
Many residents feel that democratically elected president Viktor Yanukovych, who fled the country in February, was ousted illegally through street protests. To them, the interim government in Kiev that replaced him does not represent people in the mainly Russian-speaking east and is in league with Ukrainian ultranationalists.
Also on Thursday, the Kremlin announced that Mr. Putin will join President Barack Obama and European leaders in France next month for a ceremony marking the 70th anniversary of the D-Day invasion that hastened the end of World War II.
The June 6 commemoration would mark the first time Mr. Putin and Western leaders have come face-to-face since the outbreak of the crisis in Ukraine. The U.S. and Europe have condemned Russia's provocations, ordering sanctions on Mr. Putin's inner circle and cutting Russia's ties to some international organizations.
Still, leaders from Germany and France publicly welcomed Mr. Putin's decision to attend the observance at Normandy.
The Associated Press contributed.