A pro-Russian separatist in the eastern Ukrainian town of Slaviansk (Reuters photo)
Ukraine's presidential candidates will hold their final rallies Friday as pre-election clouds darkened after the army lost 17 soldiers in attacks by pro-Russian separatists in the country's east.
Thursday's attacks in the eastern industrial belt near the Russian border underscored the difficulties of the embattled Kiev government in resolving a crisis that is threatening to tear the country apart.
Prime Minister Arseniy Yatsenyuk accused Russia of trying to "escalate the conflict" and disrupt Sunday's presidential vote, calling on the UN Security Council to hold an urgent meeting on the crisis.
He said the Kremlin's announcement of a troop withdrawal from the border was merely a "bluff" and that even if soldiers were redeploying, Ukraine was still being infiltrated by "armed terrorists".
Adding to the chaos in the east, armed separatists seized four coal mines in the first such confirmed attack on the main economic engine of the former Soviet republic.
Watch video: Pro-Russian rebels clash with Ukrainian military
Western governments have pressured Russia not to meddle in the snap election, seen as crucial to preventing all-out civil war erupting on Europe's eastern flank.
Russia set Western nerves on edge when it massed some 40,000 troops on the border, raising fears of an invasion into eastern Ukraine after its seizure of the Black Sea peninsula of Crimea in March.
On Thursday, Russia's defence ministry said four trains and more than a dozen planes were taking troops and equipment away from the border in what President Vladimir Putin said were measures to create "favourable conditions" for Sunday's election.
NATO chief Anders Fogh Rasmussen said he had seen some evidence of limited Russian troop activity that he hoped was "the start of a full and genuine withdrawal".
But rebels in Ukraine's Russified eastern industrial regions of Donetsk and Lugansk are showing no signs of scaling back resistance to what they and the Kremlin regard as an illegitimate government in Kiev.
The Ukrainian defence ministry said the worst of the two overnight attacks saw the insurgents blow up a military vehicle after volleying mortar shells and grenades at a roadblock set up by government troops near the Donetsk town of Volnovakha.
The health ministry said 16 had been killed in Volnovakha, while the defence ministry said another soldier was killed in a similar strike near Rubizhne in Lugansk.
Putin's grudging support
Kiev's interim government launched its "anti-terrorist" operation in mid-April aimed at crushing the rebels who have seized more than a dozen eastern cities and towns, and declared sovereignty in Donetsk and Lugansk.
An AFP toll compiled through UN and Ukrainian government sources puts the number of deaths across the east since then at well over 140.
Putin -- his government at risk of deeper sanctions from Washington and its European allies -- denies a direct role in the insurgency and has so far refrained from recognising the rebel republics.
But Putin rejects the legitimacy of the pro-Western team that toppled a Moscow-backed president in February on the back of a massive wave of street protests.
He has given only grudging backing to an election that is all but certain to bring to power a pro-Western president who will seek to fold the nation of 46 million more fully into Europe and break its historic dependence on Russia.
Sunday's poll pits the overwhelming favourite Petro Poroshenko -- a 48-year-old confectioner whose chocolate factories have been shuttered in Russia on controversial health and safety grounds -- against nearly 20 challengers including the divisive nationalist ex-premier Yulia Tymoshenko.
A new poll Thursday gave Poroshenko almost 45 percent of the vote against 7.5 percent for Tymoshenko, but not enough to avoid a second-round runoff on June 15.
"What is important is not the election itself," Putin said. "What is important is that (Kiev) repairs relations with the regions so that people start feeling like full-fledged citizens again."
Ukraine's interior ministry said it was mobilising 55,000 police and 20,000 volunteers to ensure security for Sunday's ballot although officials acknowledge it will be difficult to organise in the east.
Rebel leaders have vowed to disrupt polling in Donetsk and Lugansk, the heartland of Ukraine's Soviet-era industrial rust belt that churns out more than 15 percent of the country's economic output.
On Thursday, the self-proclaimed leader of Lugansk Valery Bolotov proclaimed martial law in the region and called for Putin to send peacekeeping forces that could help avert a "humanitarian catastrophe".