At least 14 soldiers died in a dawn attack on a checkpoint in eastern Ukraine.
This is the worst loss of life for Ukrainian government forces to date.
Heavily armed militants attacked the checkpoint in the Volnovakha area, in one of four attacks reported overnight in eastern Ukraine.
The attacks come just three days before Ukraine’s presidential election.
At least 14 soldiers died in a dawn attack on a checkpoint in eastern Ukraine (photo AP)
Ukraine’s interim PM Arseny Yatseniuk has called for an emergency session of the UN Security Council, saying he has evidence of Russian involvement in the violence.
Russia appears to be withdrawing troops from its border with Ukraine, easing fears of a military intervention like in Crimea in March.
NATO chief Anders Fogh Rasmussen said in a tweet that Russian troop activity near the Ukraine border might suggest that some Russian forces were preparing to pull back.
Russian President Vladimir Putin ordered troops near the border to withdraw to their bases earlier this week. Correspondents say removing the troops – estimated to number 40,000 – could help de-escalate the Ukraine crisis.
Ukrainians go to the polls on Sunday to choose a new president but voting in Donetsk and Luhansk has been seriously disrupted by the insurgency there.
The election was called after the last elected President, Viktor Yanukovych, was deposed in February amid mass protests against his pro-Russian policies.
Photographs taken by an Associated Press news agency crew show bodies lying in a field or inside a car outside the village of Blahodatne, which is near the town of Volnovakha.
Three charred armored infantry vehicles, their turrets blown away by powerful explosions, and several burned lorries could also be seen.
Residents told AP the attackers had used an armored bank lorry which the unsuspecting Ukrainian soldiers had waved through the checkpoint. The attackers then reportedly shot down the soldiers at point-blank range.
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Ukrainian soldiers sent to reclaim locations seized by pro-Russian militia in recent days, have been seen in the centre of Kramatorsk.
Kramatorsk’s airfield was recaptured on Tuesday after Ukraine’s acting President Oleksandr Turchynov announced an “anti-terror operation”.
Armored personnel carriers in the town were blocked by pro-Russian locals who were talking to the soldiers.
Tension is running high, with Russia’s President Vladimir Putin warning Ukraine is “on the verge of civil war”.
Vladimir Putin made the comments in a phone call to German Chancellor Angela Merkel, after President Oleksandr Turchynov announced the start of the operation on Tuesday.
Russian officials said both leaders “emphasized the importance” during the conversation of planned four-way talks on Thursday between senior diplomats from Russia, the EU, the US and Ukraine.
Ukrainian soldiers sent to reclaim locations seized by pro-Russian militia in recent days, have been seen in the centre of Kramatorsk
However, Ukrainian and Western officials have accused Russia of being behind the pro-Russia activism in the region. Moscow strongly denies fomenting the unrest.
Ukrainian counter-terrorism chief General Vasyl Krutov, speaking to reporters at Kramatorsk airfield late on Tuesday, emphasized that armed individuals had crossed the border into Ukraine.
“We have to deal with a very serious, highly skilled and very professional opponent,” he said according to Interfax-Ukraine news agency.
“They display a very high level of preparedness, tactical and practical skill. These people must have been to hot spots across the world and on their own territory.”
Last month’s annexation of Crimea by Moscow was preceded by the presence of uniformed, armed men believed to be Russian troops, although in greater numbers than the similarly dressed and equipped men who have been seen in eastern Ukraine, correspondents say.
The Kremlin has condemned the Ukrainian military operation in eastern Ukraine as an “anti-constitutional course to use force against peaceful protest actions”.
Russia’s foreign ministry has expressed “deep concern” at reports of casualties in eastern Ukraine, but these cannot be confirmed.
Tensions have mounted in recent days after pro-Russian rebels seized buildings in about 10 towns and cities across eastern Ukraine.
They are demanding greater autonomy or referendums on secession from Ukraine.
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