BLAHODATNE, Ukraine -- In the deadliest raid yet on Ukrainian troops, pro-Russia insurgents attacked a military checkpoint Thursday, killing 16 soldiers, and the interim prime minister accused Moscow of trying to disrupt the upcoming election for a new president to lead the divided country out of its crisis.
A rebel commander said one of his fighters also died in the raid in eastern Ukraine, which left a gruesome scene of charred military vehicles and scorched bodies near the town of Volnovakha, 20 miles south of the city of Donetsk.
Witnesses, including a medical worker, said more than 30 Ukrainian troops were wounded, with some in grave condition. Fighting also raged in at least two other villages.
The armed uprising and the government's offensive to put it down have cast a shadow over Sunday's election, with Kiev acknowledging it will be impossible to hold the vote in some areas. In the eastern regions of Donetsk and Luhansk, where separatists have declared independence and pledged to derail the vote, election workers reported threats and interference from gunmen.
Interim Prime Minister Arseniy Yatsenyuk accused Russia of escalating the conflict and trying to disrupt the election. In a post on Facebook, he called for an urgent session of the U.N. Security Council and said Ukraine would present evidence of Moscow's involvement.
In the attack on the checkpoint, residents said the rebels arrived in an armored bank truck, which the unsuspecting Ukrainian soldiers waved through. The rebels opened fire with automatic weapons and rocket-propelled grenades, acting President Oleksandr Turchynov said.
The Donetsk regional administration and a Health Ministry official said 16 soldiers were killed. Associated Press journalists saw 11 dead soldiers in the field near the village of Blahodatne, outside Volnovakha.
Ukraine's Defense Ministry said the attackers hit an ammunition section in one of the military vehicles, which exploded in a fireball. Three blackened armored infantry vehicles, their turrets blown away, and several burned trucks stood at the site. Bodies apparently burned by the explosion and fire were scattered nearby.
A leading rebel commander claimed responsibility for the raid. In the courtyard of the occupied police headquarters of the town of Horlivka, he displayed seized Ukrainian weapons and the passports of what he said were two of the dead soldiers. He also provided coordinates about the location of the attack shortly after it had occurred. There was no way to confirm his claim independently.
"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.
Pavel Gubarev, a representative of the separatist Donetsk People's Republic, denied its men had carried out the attack, however. Gubarev, who does not appear to have much influence with the separatist group, appeared on Russian TV and accused Ukrainian nationalists from the group Right Sector of firing on Ukrainian soldiers because they had refused to attack civilians.
Many in the east resent the government in Kiev, seeing it as led by nationalists bent on repressing the Russian-speaking minority. The interim government came to power in February after months of street protests chased pro-Russian President Viktor Yanukovych from power.
But many in the rebellious regions also have grown increasingly exasperated with the insurgents, whom they blame for putting civilians in the crossfire.
In the village of Semenovka on the outskirts of Slovyansk, shelling badly damaged several houses. Zinaida Patskan had her roof torn away by an explosion that also shattered a wall. She said she hid under a kitchen table with her cat, Timofey, in the shelling.
"Why they are hitting us?" the 80-year-old said, bursting into tears. "We are peaceful people!"
About 100 Semenovka residents later vented their anger against the central government, demanding Ukrainian forces end the offensive and withdraw. Speakers at a rally also urged a boycott of the presidential vote.
In the Luhansk region, sustained gunfire and mortar fire rocked the town of Lysychansk. One shell hit a house, which burst into flames. A police duty officer at the town hospital said one fighter had died and five were wounded.
In the evening, a convoy of at least 200 Ukrainian soldiers was seen traveling to Slovyansk from the north. The soldiers were in two armored vehicles and eight military trucks, one of the larger concentrations seen in recent weeks.
While the fighting raged, Russia's Defense Ministry said its forces were leaving the regions near Ukraine as part of a 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.
Gen. Philip Breedlove, NATO's supreme commander in Europe, said 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 rejected Yatsenyuk's claims of Russian interference as unfounded and denounced his call for a U.N. meeting as "propaganda."
Putin's pullout order and his remarks welcoming Ukraine's election reflected an attempt to ease tensions with the West over Ukraine and avoid a new round of U.S. and Europea Union sanctions. He has ignored the plea of some of the rebels to join Russia.
The U.S. and the EU imposed travel bans and asset freezes on members of Putin's inner circle after Russia annexed Crimea in March. The West warned that crippling sanctions against entire sectors of the Russian economy could follow if Moscow tries to grab more land or attempts to derail the election.
Russia wants guarantees that Ukraine will not join NATO and has advocated for constitutional reforms that would give broader powers to Ukraine's regions. That would maintain Moscow's clout in Ukraine's industrial heartland.
The USS Vella Gulf, a U.S. Navy guided missile cruiser, will move into the Black Sea Friday as part of an effort by the U.S. to maintain assets in the region to reassure American allies in the wake of the unrest.