Last Updated May 22, 2014 2:30 PM EDT
BLAHODATNE, Ukraine -- Just days before Ukraine holds a presidential election, pro-Russia insurgents attacked a military checkpoint in the east Thursday, killing 13 troops in the deadliest raid yet in weeks of fighting, Ukraine's leader said.
A rebel commander who claimed responsibility for the attack said one of his men also was killed.
Acting Prime Minister Arseniy Yatsenyuk accused Russia of escalating the conflict in Ukraine's eastern regions and trying to disrupt Sunday's election. In a posting on Facebook, he called for an urgent session of the U.N. Security Council and said Ukraine would present evidence of Russia's involvement.
The rebels attacked a checkpoint near the town of Volnovakha, firing automatic weapons and rocket-propelled grenades, acting President Oleksandr Turchynov said. The Ukrainian Defense Ministry said the attackers hit an ammunition section in one of the vehicles, which exploded in a fireball.
AP journalists saw 11 dead Ukrainian soldiers scattered in a field outside Volnovakha, 30 kilometers (20 miles) south of the major eastern city of Donetsk.
Witnesses including a medical worker said more than 30 other Ukrainian troops were wounded in the attack and some of them were in grave condition. All the wounded were being treated at nearby medical facilities.
The carnage cast a shadow over Ukraine's upcoming presidential election, which authorities in Kiev had hoped would defuse tensions and stabilize the country. Separatists in the eastern Donetsk and Luhansk regions, however, have declared independence from the government in Kiev and have pledged to derail the vote. Election officials and voters have faced intimidation and sometimes death threats from the rebels in those areas and some residents have called for a boycott of the presidential vote.
Kiev authorities now admit it will be impossible to stage the vote in some eastern areas.
Three charred Ukrainian armored infantry vehicles, their turrets blown away, and several burned trucks stood at the site of Thursday's attack. Scorched bodies, apparently burned by the explosion and fire, were scattered near the vehicles.
Residents said attackers used an armored bank truck, which the unsuspecting Ukrainian soldiers waved through, and then mowed them down at point-blank range. Their account couldn't be independently confirmed.
In the town of Horlivka, a leading rebel commander claimed responsibility for the raid. He showed off an array of seized Ukrainian weapons and provided specific coordinates about the location of the attack. There was no way to independently confirm his claim, however.
"We destroyed a checkpoint of the fascist Ukrainian army deployed on the land of the Donetsk Republic," said the commander, who wore a balaclava and insisted he be identified by his nom de guerre, "Bes," Russian for "demon." He said one of his men also was killed.
"The weapons you see here have been taken from the dead, they are trophies," the rebel commander said, showing automatic and sniper rifles, rocket grenade launchers and bulletproof vests in the courtyard of the occupied Horlivka police headquarters.
"People living in western Ukraine: Think about where you are sending your brothers, fathers and sons, and why you need any of this," he added.
A representative of the separatist Donetsk People's Republic, however, denied that its men had carried out the attack. Speaking on Russian television, Pavel Gubarev accused Ukrainian nationalists from the Right Sector group of firing on Ukrainian soldiers because they had refused to attack civilians.
The comments appeared questionable, however. Gubarev does not appear to have much influence with the armed separatist rebel group.
Many in the east resent the government in Kiev, which came to power after a pro-Russian president fled in February following months of protests, seeing it as led by nationalists bent on repressing Russian-speakers. But many local residents also have grown increasingly exasperated with the rebels, whom they blame for putting civilians in the crossfire.
In the village of Semenovka on the outskirts of Slovyansk, artillery shelling badly damaged several houses Thursday.
Zinaida Patskan, 80, had her roof torn away by an explosion that also shattered a wall. She said she was hiding under a kitchen table with her cat, Timofey, when the shelling came.
"Why they are hitting us?" she said, bursting into tears. "We are peaceful people!"
About 100 Semenovka residents later vented their anger against the central government, demanding that Ukrainian forces cease their offensive against the separatists and withdraw from the region. Speakers at the rally also urged residents to boycott the presidential vote.
In the eastern Luhansk region, sustained gunfire and shelling rocked the town of Lysychansk. One mortar bomb hit a house, which burst into flames.
While the fighting raged in Ukraine, Russia's Defense Ministry said Thursday its forces were leaving the regions near Ukraine as part of a massive military pullout ordered by President Vladimir Putin. It said four trainloads of weapons and 15 Il-76 heavy-lift transport planes had already left the Belgorod, Bryansk and Rostov regions.
NATO had estimated Russia had 40,000 troops along the border with Ukraine.
Gen. Philip Breedlove, NATO's supreme commander in Europe, told reporters in Brussels that some Russian military movements had been detected but it was too early to assess their size or importance. He said a very large and capable Russian force still remained close to Ukraine.
In Kiev, Yatsenyuk described Russia's announcement as a bluff.
"Even if the troops are withdrawing, Russian authorities are still assisting the armed terrorists who were trained in Russia," he said.
Russian Foreign Ministry spokesman Alexander Lukashevich on Thursday rejected Yatsenyuk's claims of Russian interference in the east as unfounded and denounced his call for a U.N. Security Council meeting as "propaganda."
Putin's pullout order and his remarks welcoming Ukraine's presidential election reflected an attempt to ease tensions with the West over Ukraine and avoid a new round of Western sanctions. He has ignored the plea of some of the rebels in eastern Ukraine to join Russia.
The United States and the European Union have imposed travel bans and asset freezes on members of Putin's entourage after Russia annexed Crimea in March. The U.S. and EU have warned that more crippling sanctions against entire sectors of the Russian economy could follow if Russia tries to grab more land from Ukraine or attempts to derail Ukraine's election.
Russia has pushed for guarantees that Ukraine will not join NATO and has advocated for constitutional reforms that would give broader powers to Ukraine's regions, which would maintain Moscow's clout in Ukraine's industrial heartland.
The USS Vella Gulf, a Navy guided missile cruiser, will move into the Black Sea on Friday as part of U.S. efforts to reassure America's allies in the region in the wake of the unrest in Ukraine.