Ukraine's electoral body has voiced fears that it may be impossible to hold next weekend's presidential poll in the east, where a pro-Moscow insurgency is threatening to plunge the country into a civil war.
The Central Election Commission said on Saturday it could not prepare for the vote in the region because of threats and "illegal actions" by separatists who have overrun more than a dozen towns and cities since early April, the news agency AFP reported.
It called for the authorities in Kiev to take urgent action to ensure security, saying that violence could prevent almost two million people from voting on May 25, when about 36 million overall will be called to the polls.
The warning came as Ukraine's embattled government held a second round of "national unity" dialogue under a Western-backed plan to try to defuse the crisis, but failed to report any progress.
Russia said Kiev must first halt its month-long offensive against the rebels, questioning the legitimacy of an election held under "the thunder of guns".
Separatist leaders 'not invited'
While Saturday's talks in the eastern city of Kharkiv included a broad spectrum of Ukrainian politicians, including pro-Russians, no separatist leaders were invited.
"Round tables are beautiful things but they won't solve anything," said Aleksandr Borodai, "prime minister" of the rebels' self-declared People's Republic of Donetsk, one of the rebel-held areas in the east.
The West sees next Sunday's election as crucial to defusing the crisis and preventing the recession-hit country from disintegrating further after Russia's annexation of Crimea.
Despite a month-long offensive, the Ukrainian military has failed to wrest back control of the main industrial regions of Donetsk and Lugansk, where rebels have declared their own independent republics in defiance of Kiev and the West.
"Can elections held amid the thunder of guns really meet the democratic norms of the electoral process?" Russia's foreign ministry said in a statement, urging Kiev to immediately halt "punitive action against its own citizens".
Moscow said Kiev was using the unity talks "as a cover for aggressive action" and urged Western nations to tell Kiev to "launch real and not phony work towards national reconciliation".
At Saturday's round-table, interim Prime Minister Arseniy Yatsenyuk said the government was in favour of giving eastern Ukraine more financial autonomy but again ruled out the idea of federalism - a policy favoured by Russia.
"Ukrainians, please, use words not weapons," pleaded Organisation for Security and Cooperation in Europe (OSCE)-appointed mediator Wolfgang Ischinger.
The West has kept up the pressure on Moscow to allow the election to go ahead.
In a phone call on Friday, US President Barack Obama and French counterpart Francois Hollande "underscored that Russia will face significant additional costs if it continues its provocative and destabilising behaviour".
Obama has already drafted an executive order for sanctions across key sectors such as banking, energy, defence and mining, adding to punitive measures already imposed by Washington and Brussels.
The election is expected to deliver victory to billionaire chocolate baron Petro Poroshenko, who enjoys a huge lead over former prime minister Yulia Tymoschenko.