KIEV (Reuters) - Ukraine said more than a dozen servicemen were killed on Thursday in an early morning clash with pro-Russian separatists, fuelling security concerns ahead of a presidential election on Sunday seen as crucial for its fragile democracy.
Kiev's pro-Western government said its forces had also rebuffed an attempt by the separatists to enter its territory from Russia, and it called for an emergency session of the U.N. Security Council to discuss Moscow's role in the violence.
The government hopes the election will stabilize Ukraine after mass street protests toppled Moscow-backed president Viktor Yanukovich in February, but the separatists have vowed to prevent the poll going ahead in eastern towns where they have seized control.
The United States and European Union say they will impose broad sanctions on Russia if it tries to derail the election, billed as the country's most important since the end of communism in 1991.
Moscow, which annexed Ukraine's Crimea peninsula in March after Yanukovich's fall, denies arming or training the rebels.
"The aim (of the separatist fighters) was to attack and seize the town of Volnovakha. Our troops defended the town ... but regrettably, under fire from mortars and grenade launchers and heavy gunfire, our boys were killed," Interfax news agency quoted acting President Oleksander Turchinov as saying.
Turchinov said 13 servicemen had been killed. Authorities in the Donetsk region, where the attack took place, put the death toll at 16, without specifying whether they were all soldiers or also included separatists, and said 32 people were injured.
Top Ukrainian security official Andriy Parubiy said he expected more separatist violence in the coming days "because their whole concept is aimed at disrupting the presidential election".
"I would like to appeal to all citizens of Ukraine, not only to those in the east: on Sunday ... we must all go and vote ... Going to the elections, holding the elections means defeating Putin," Parubiy told a news conference.
Opinion polls suggest confectionery magnate Petro Poroshenko, a former ally of Yanukovich who later joined the protests against him and supports Ukraine's tilt towards the West, will win the election, possibly in Sunday's first round.
In the eastern region of Luhansk, Ukrainian border guards repelled an incursion by dozens of separatists armed with grenade launchers and rifles, the border service said. Several border guards were injured in the incident.
Ukraine's energy ministry said there had been "attacks by terrorists" on several coal mines in eastern Ukraine, forcing a suspension of production there.
Also on Thursday, armed representatives of the self-proclaimed 'Donetsk People's Republic' abducted the head of an election commission in the city of 1 million people and seized a voters' register and a stamp, local officials said.
Other election commissions in Donetsk have already been forced to quit. A top U.N. human rights official this week expressed concern over reports of abductions and intimidation of election officials in eastern Ukraine.
Interim Prime Minister Arseny Yatseniuk said Kiev was ready to submit evidence of what he called Moscow's attempts "to escalate the conflict" to an emergency session of the U.N. Security Council, of which Russia is a permanent member.
Moscow, for its part, accused Kiev of stepping up military operations in eastern Ukraine and of failing to implement measures aimed at ending the crisis.
NATO says Russia has amassed tens of thousands of troops across the border from eastern Ukraine.
On Thursday, Moscow announced it had moved some troops and military equipment from the Ukraine border area, but Yatseniuk dismissed it as a "bluff", and NATO's top military commander said Russia's forces in the region remained "very large".
Kiev says Sunday's election cannot be held in parts of the Donetsk and Luhansk regions and that Moscow is deliberately seeking to undermine Ukrainian democracy, a charge echoed by the United States and the EU.
Russia denies the legitimacy of the current Kiev government and has asserted its own right to intervene on behalf of Russian speakers outside Russia's borders.
Election front-runner Poroshenko has urged voters to hand him victory in Sunday's first round of voting, suggesting that Ukraine's deteriorating security situation might otherwise derail the election before a second round can be held.
If no candidate wins more than 50 percent of the vote on Sunday, a second round will be held on June 15.
A poll watchdog has said it expects a turnout of at least 70 percent nationwide, despite the loss of Crimea and the turmoil in the east.
Additional reporting by Pavel Polityuk in Kiev and Sabina Zawadzki in Donetsk; Writing by Gareth Jones; editing by Philippa Fletcher and Will Waterman