Kiev, Ukraine (CNN) -- Sixteen people were killed an attack overnight on soldiers in Ukraine's eastern Donetsk region, local health officials said, in the latest spike in violence ahead of the weekend's presidential election.
At least 13 of those who died in Donetsk were soldiers, according to acting President Oleksandr Turchynov.
Another soldier was killed in a strike on a military convoy in the Luhansk region, the Defense Ministry said earlier, bringing the death toll overnight to at least 14 Ukrainian troops.
Donetsk region's Health Department said 16 people had died in total and that 32 were hospitalized across the region, but did not specify how many were soldiers.
In the Donetsk attack, an armored vehicle at a checkpoint in Volnovaha was hit by rocket-propelled grenades and mortars, which caused ammunition on board to ignite, the Defense Ministry said.
The second attack hit a convoy of vehicles in Rubizhne in the Luhansk region, the ministry said. It added that "terrorists" blocked a bridge with "civilian provocateurs" and then opened fire using the people as human shields.
No independent confirmation of the claims or response from separatist forces in eastern Ukraine was immediately available.
Ukrainian Prime Minister Arseniy Yatsenyuk called for an immediate emergency meeting of the U.N. Security Council following the reported violence, which came as the interim government in Kiev prepares to hold presidential elections Sunday.
"We will provide evidence that it is Russia trying to escalate the conflict, attempts to seize Ukrainian state border checkpoints and scuttle the presidential election," he was quoted as saying by his office.
Tensions remain high in Donetsk and Luhansk, where pro-Russia separatists staged a referendum on independence this month and control key public buildings in a number of towns and cities.
Ukraine's Central Election Commission said Wednesday that 13 out of 34 local election commissions in Luhansk and Donetsk were "blocked," or under the control of militia, the official Ukrinform news agency reported.
Three others "are operating but under threat of seizure," it said.
NATO chief: Signs of Russian troop activity
Kiev and the West have accused Russia of supporting the separatists and have urged Moscow to take steps to de-escalate the crisis.
NATO Secretary General Anders Fogh Rasmussen said Thursday that there were indications that Russia may be preparing to pull back some of the forces it has amassed near Ukraine's border.
On Monday, the Kremlin said that Russian President Vladimir Putin had ordered tens of thousands of troops near the Ukraine border to return to their bases but added the withdrawal could take some time.
NATO has seen "limited" Russian troop activity "that MAY suggest that some of these forces are preparing to withdraw," Rasmussen said via Twitter. "It is too early to say what this means, but I hope this is the start of a full and genuine withdrawal."
His remarks came a day after a U.S. defense official told CNN the United States had seen the first preliminary signs that Russian forces might be preparing to move away from Ukraine's eastern border.
Rasmussen cautioned that most of the Russian force deployed near the border remains there and that Russian military exercises continue.
But he added, "Any meaningful, comprehensive and verifiable withdrawal would be a first step from #Russia into the right direction."
Yatsenyuk was more skeptical, dismissing Russia's claims of a pullback as a "bluff," according to a statement from his office.
"Even if the troops are pulling back, armed terrorists trained in Russia illegally break the state border of Ukraine with the direct assistance of Russia," he said.
Ukraine cites 'provocations'
Ukraine's Foreign Ministry said Thursday that Ukrainian border guards had again stopped armed militant groups trying to bring weapons and ammunition across the border from Russia.
These attempts occur with "the full connivance of the Russian authorities and unlawful inactivity by Russian border guards," it said in a statement.
"These and other provocations by Russian side in Ukraine are regarded as attempts to disrupt the presidential elections on 25 May and destabilize the situation in the eastern region of our country."
Ukraine's Interior Ministry also reported armed border clashes which, it said, left five border guards injured and one suffering a concussion following a grenade blast.
Moscow, which blames the unrest roiling its neighbor on far-right, ultranationalist groups, denies having direct influence over the pro-Russia militants in Ukraine.
NATO and the United States have previously estimated the size of the Russian force gathered near the border with Ukraine at around 40,000 troops.
The United States, which along with other Western countries has sanctioned Russia for its disputed takeover of Ukraine's Crimea region, has threatened additional punishment for Russia if it fails to pull its troops back.
Diplomats meet over Prince Charles report
Separately, Russia's deputy ambassador to the United Kingdom is meeting a senior UK Foreign Office official Thursday in London amid controversy over comments reportedly made by Prince Charles.
The Russian Embassy requested the meeting Wednesday, two days after Charles reportedly compared Putin to Adolf Hitler during a trip to Canada.
A statement will be given after the meeting, the Foreign Office said.
According to the Daily Mail newspaper, Charles made the offhand comment during a tour of the Canadian Museum of Immigration in Halifax, Nova Scotia, while talking with a Polish woman who told the British royal how she'd escaped the Nazi Holocaust by fleeing to Canada.
"The prince then said, 'And now Putin is doing just about the same as Hitler,' " the newspaper quoted the woman as saying, referring to Russia's annexation of Crimea in March.
CNN has not been able independently to confirm the conversation.
Russian Foreign Ministry spokesman Alexander Lukashevich said that "if these words were really said, they undoubtedly do no honor to the future British monarch," according to the ministry's website.
"We find the use of members of the British royal family by Western media in a propaganda campaign against Russia on the difficult international issue of the situation in Ukraine unacceptable, outrageous and low," Lukashevich is quoted as saying.
Representatives for Charles declined to comment.
"However, we would like to stress that the Prince of Wales would not seek to make a public political statement during a private conversation," Clarence House said in a statement.
Journalist Victoria Butenko reported from Kiev, and CNN's Laura Smith-Spark wrote from London. CNN's Barbara Starr, Lindsay Isaac, Mike Pearson and Neda Farshbaf contributed to this report.