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, 30 kilometres 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 UN 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 armoured 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.

AP journalists saw 11 dead Ukrainian soldiers scattered in a field near the village of Blahodatne, outside Volnovakha, 30 kilometres south of the major city of Donetsk.

Witnesses including a medical worker said over 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 Ukrainian Defence Ministry said the attackers hit an ammunition section in one of the vehicles, which exploded in a fireball.

Three charred Ukrainian armoured infantry vehicles, their turrets blown away, and several burned trucks stood at the road site in Blahodatne. Scorched bodies, apparently burned by the explosion and fire, were scattered near the vehicles.

Canadian observers to monitor vote

Former Ontario premier Mike Harris is in Kyiv as co-leader of the 500 Canadians who will observe the Ukrainian presidential election. About 2,000 observers are arriving from around the world, and Canadians are the largest group.

Ukrainians "want to have a fair election," Harris told CBC correspondent Nahlah Ayed in Kyiv.

"I don’t want to prejudge any outcomes; we're not here to do that," he said. "What we're here to judge is, is this [a] fair, open election where people are able to express freely their democratic right."

He added that the safety of observers is paramount. Some will be deployed to what he described as "areas of concern," where they are "expecting some challenges," in an apparent reference to the eastern region.

Vote impossible in some eastern areas

Authorities in Kyiv see the vote as a chance to defuse tensions and stabilize the country. Even so, they have admitted it will be impossible to stage the vote in some eastern areas where election officials and voters have faced intimidation and sometimes death threats from the rebels.

In the town of Horlivka, a masked rebel commander claimed responsibility for the checkpoint attack and showed an array of seized Ukrainian weapons.

Ukraine soldier

A Ukrainian soldier reacts at a gunfight site near the village of Blahodatne, eastern Ukraine, that killed at least 11 Ukrainian troops and wounded more than 30 others. (Ivan Sekretarev/AP)

"We destroyed a checkpoint of the fascist Ukrainian army deployed on the land the Donetsk Republic," said the commander, who wore a balaclava and identified himself 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.

Many in the east resent the government in Kyiv, which came to power after a pro-Russian president fled in February following months of protests, seeing it as nationalists bent on repressing Russian-speakers. But many locals also have grown increasingly exasperated with the rebels, whom they blame for putting civilians in the crossfire.

'We are peaceful people!'

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.

While fighting raged in Ukraine, Russia's Defence 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 has 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.

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A pro-Russian separatist shows an ammunition round at a checkpoint in east Ukraine. (Maxim Zmeyev/Reuters)

In Kyiv, Yatsenyuk described Russia's announcement of troops pull-out as "bluffing."

"Even if the troops are withdrawing, Russian authorities are still assisting the armed terrorists who were trained in Russia," he said.

Russia's Foreign Ministry spokesman Alexander Lukashevich on Thursday rejected Yatsenyuk's claims of Russian interference in the east as unfounded.

Putin's pullout order and his remarks welcoming Ukraine's presidential election Sunday reflected an apparent 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 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 constitutional reforms that would give broader powers to Ukraine's regions, which would maintain Moscow's clout in the Russian-speaking eastern regions that form the nation's industrial heartland.

With files from CBC News