Russian President Vladimir Putin said on Wednesday that Moscow had withdrawn troops from the border with Ukraine. Sergei Karpukhin / Reuters

The councils of Donetsk and Lugansk in southeastern Ukraine won't postpone referendums on their regions' future as part of Ukraine and will hold them as planned on Sunday, the cities' anti-government activists said.

"This is not our decision, this is the decision of people of Donbas region," said Andrey Purgin, one of the leaders of the self-proclaimed Donetsk People's Republic, according to Russia Today. "People of Donbas got their chance to make a heroic deed and we can't deprive them from this chance."

The decision was approved by the city's council.

A spokeswoman for insurgents in the flashpoint town of Slovyansk confirmed to AFP that the vote would happen on Sunday.

Previously, Russian President Vladimir Putin called to delay the voting.

While Putin's call on Wednesday to postpone the vote was seen as part of an effort to step back from confrontations with the West, he oversaw military exercises on Thursday that Russian news agencies said simulated a massive retaliatory nuclear strike in response to an enemy attack.

Putin said the exercise involving Russia's nuclear forces had been planned back in November, but it came as relations between Russia and the West have plunged to their lowest point since the Cold War, according to the Associated Press.

Ukraine has deployed a 15,000-strong military force near Russia's borders, said Russian Deputy Defense Minister Anatoly Antonov.

"The 15,000-strong grouping of Ukrainian forces has been deployed in the border areas. Military conscription has resumed (in Ukraine)," he said. "At the same time, NATO amasses its grouping of forces in Eastern Europe," according to ITAR-Tass.

Antonov said such actions are not contributing to the efforts to de-escalate tensions in Ukraine considering the ongoing crisis in the country.

He said Russia has called on official representatives from NATO and the Pentagon "to quit cynically deluding the international society on the real state of affairs on the Russian-Ukrainian border".

On the ground in Ukraine, many fear that the referendum could be a flashpoint for further violence between Ukrainian troops and the pro-Russia militants who have seized government buildings in about a dozen cities in the east.

Organizers of the weekend referendum have said the vote would decide whether to give the eastern regions more autonomy within Ukraine, but they have left open the possibility of using it to seek independence or annexation by Russia.

Putin also declared on Wednesday that Russia has pulled its troops away from the Ukrainian border, although NATO and Washington said they saw no signs of this.

Putin also spoke more positively about the interim Ukrainian government's plan to hold a presidential election on May 25, calling it a "step in the right direction", but reiterated Russia's long-standing contention that it should be preceded by constitutional reforms.

His spokesman, Dmitry Peskov, added on Thursday that the election could only be considered legitimate if Ukraine stops its "punitive operations" in the east and begins a national dialogue on resolving the crisis, the Interfax News Agency reported.

According to the BBC, the Kiev government said the planned referendum is illegal. It also reported that supporters of the government in Kiev have launched their own campaigns in Donetsk and Lugansk "to counter the narrative put out by the separatists".

A poll by the Washington-based Pew Research Center released on Thursday showed that a strong majority of Ukrainians want their country to remain a single, unified state and this was true even in the largely Russian-speaking east where the pro-Russia insurgency has been fighting for autonomy.

AP-AFP-China Daily