Russia also questioned how an election taking place under the "thunder of guns" could possibly meet democratic norms and demanded that Kiev halt its offensive against the rebels.
The Central Election Commission called for the authorities in Kiev to take urgent action to ensure security in the east, saying that violence could prevent almost two million people from voting on May 25.
Its warning came as Ukraine's embattled Western-backed government began a second round of "national unity" dialogue called for under an OSCE-sponsored peace plan.
But again, no separatist leaders were invited to the round-table talks being held in the eastern city of Kharkiv, to Russia's chagrin.
"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.
"While our territory is occupied there will be no negotiations."
The West sees next Sunday's election as crucial to defusing the crisis on Europe's eastern flank and preventing the former Soviet republic from disintegrating further after Russia's annexation of Crimea.
But the 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.
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 now 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 asked in a statement, urging Kiev to immediately halt "punitive action against its own citizens".
It also said the government 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".
But the United States and its allies are piling further pressure on Moscow to allow the election to go ahead.
In a phone call on 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", the White House said in a statement.
Obama has already drafted an executive order for sanctions across key sectors in Russia such as banking, energy, defence and mining, adding to punitive measures already imposed by Washington and Brussels.
In total, 36 million Ukrainians are eligible to vote on May 25 in an election expected to deliver victory to billionaire chocolate baron Petro Poroshenko.
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 new national leadership 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 using air support and heavy artillery.
In further violence on 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 on Friday that 127 people overall had been killed in the southeast and sounded the alarm over the deteriorating human rights situation.
UN rights chief Navi Pillay listed a series of "targeted killings, torture and beatings, abductions, intimidation and some cases of sexual harassment".
She called on those with influence on the armed groups in the east "to do their utmost to rein in these men who seem bent on tearing the country apart".
Pillay also voiced deep concern about harassment and intimidation of ethnic Tatars in the Crimean peninsula, which was annexed by Russia in March in the face of international outrage.
But Moscow blasted the report, complaining about its "complete lack of objectivity, blatant discrepancies and double standards".
Aside from the risk of civil war on its doorstep, the crisis has stoked concerns 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.
Nearly 15% of all gas consumed in Europe is delivered from Russia via the former Soviet republic.