A dawn attack on a checkpoint in eastern Ukraine has left at least 14 soldiers dead, in the worst loss of life for government forces to date.
Heavily armed militants attacked the checkpoint in the Volnovakha area, in one of four attacks reported overnight in eastern Ukraine.
It is unclear who attacked the checkpoint, with one Ukrainian officer telling the BBC it was not separatists.
The attacks come just three days before Ukraine's presidential election.
Ukraine's interim Prime Minister, 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 armoured 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 armoured bank lorry which the unsuspecting Ukrainian soldiers had waved through the checkpoint. The attackers then reportedly shot down the soldiers at point-blank range.
BBC journalists who reached the scene were not allowed to film by Ukrainian soldiers. The BBC's Olga Ivshina found 27 wounded survivors at the central hospital in Volnovakha.
Local people said fighting had erupted at 05:00 local time (02:00 GMT). They said they had heard explosions and gunfire, and some spoke of seeing many wounded later.
The defence ministry blamed the latest attack on "terrorists" - the term commonly used by Ukrainian officials for armed pro-Russian separatists in Donetsk and the neighbouring Luhansk region.
Speaking on condition of anonymity, an army major who spoke to the survivors of the attack told our correspondent he was sure that the attackers were not Donetsk separatists but "mercenaries".
Donetsk rebel leader Pavel Gubarev went on Facebook to deny that separatist forces had attacked the soldiers.
However, a self-styled rebel commander in a nearby town, Horlivka, told AP that his forces had carried out the assault and had seized weapons.
"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 identified himself by his nom de guerre, Bess ("demon" in Russian).
Separatists have been skirmishing with security forces in Donetsk for weeks but this is the deadliest attack on soldiers to date.
Ukrainian security chief Andriy Parubiy said four attacks had taken place overnight, three of them repelled with loss of life and one - in the Luhansk region - was still going on.
In another development, residents of a village near Sloviansk, a town in Donetsk often seen as the separatists' stronghold, complained of shelling that appeared to come from government positions.
"Why they are hitting us?'' asked Zinaida Patskan, 80, outside her ruined house in Semenovka. "We are civilians!"