Members of an election commission empty a ballot box to start counting ballots at a polling station in the eastern Ukrainian city of Lugansk, on May 11, 2014
Ukraine's election body voiced fears on Saturday 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 all-out civil war.
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".
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 on Europe's eastern flank and preventing the recession-hit country from disintegrating further after Russia's annexation of Crimea.
But the Central Election Commission said it could not prepare for the vote in the east because of threats and "illegal actions" by separatists who have overrun more than a dozen towns and cities since early April.
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 some 36 million overall will be called to the polls.
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.
'Phony' unity effort"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".
It 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, Prime Minister Arseniy Yatsenyuk said the government was in favour of giving the east more financial autonomy but again ruled out the idea of federalism - a policy favoured by Moscow.
"Ukrainians, please, use words not weapons," pleaded 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 Friday, 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.
The vote was called by the new leaders installed in Kiev after months of sometimes deadly pro-EU protests that led to the February ouster of Kremlin-backed president Viktor Yanukovych, viewed by many as corrupt and authoritarian.
Pro-Russians in the east took up arms in early April, refusing to recognise the legitimacy of a government they charge is made up of ultra-nationalists and neo-fascists.
Fighting rages almost every night, particularly around Slavyansk, the epicentre of the insurrection.
'Targeted killings, torture'Moscow accused Kiev of "creating a real threat to the lives of civilians", saying people had been wounded when government troops attempted to storm Slavyansk overnight.
In further violence Saturday, a gunfight erupted when rebels launched an assault to free the self-declared governor of Lugansk who had been detained by Ukrainian forces on his return from Russia.
The United Nations said Friday that 127 people overall had been killed in the southeast and sounded the alarm over the deteriorating human rights situation including "targeted killings", torture and abductions.
UN rights chief Navi Pillay also voiced deep concern about intimidation of ethnic Tatars in the Crimean peninsula since its annexation by Russia in defiance of international law.
Moscow blasted the report as having a "complete lack of objectivity, blatant discrepancies and double standards".
On Saturday, Polish newspaper Gazeta Wyborcza said its Moscow correspondent Waclaw Radziwinowicz was held for six hours in the Crimean capital Simferopol.
It said he was arrested at a restaurant where he was with a Ukrainian journalist and a Tatar woman while researching a story on the 70th anniversary of Stalin's deportation of the Tatars from Crimea.
The Ukraine crisis has stoked worries in Europe about energy supplies after Russian President Vladimir Putin threatened to cut the flow of gas to Ukraine if it fails to pay its bills by early June.