Ukraine's President-elect Petro Poroshenko has vowed to punish "bandits" after pro-Russian rebels shot down an army helicopter, killing 12.
"These criminal acts of the enemies of the Ukrainian people will not go unpunished," he was quoted as saying by the Unian news agency.
The helicopter was shot down near the eastern city of Sloviansk. An army general was among those killed.
Sloviansk has seen fierce fighting between the rebels and Ukraine's army.
Russia has reiterated calls for Ukraine to stop its military campaign against the pro-Moscow rebels and "start a real national dialogue".
In a separate development, Ukraine, Russia and the EU are due to open a new round of talks in Berlin on resolving a growing dispute over Moscow's gas supplies to Kiev.
Russia's Gazprom has threatened to cut off gas supplies to Ukraine if it fails to pay its huge debt.
Mr Poroshenko, who won last Sunday's presidential elections, expressed his condolences to the families of the helicopter crew.
"We must make every effort to make sure that no more Ukrainians die at the hands of terrorists and bandits," he said.
The Mi-8 helicopter was hit during heavy fighting between Sloviansk and Kramatorsk, Donetsk region, after it had dropped off troops at a military base.
The 12 people on board were killed, including Maj Gen Kulchytskiy, head of combat and special training, the Ukrainian National Guard said.
It added that the "criminals" who attacked the helicopter were later "destroyed" by Ukrainian troops involved in an "anti-terror" operation.
Two helicopters were also shot down by separatists earlier this month.
The conflict has intensified in recent days. The rebels say they lost up to 100 fighters when they unsuccessfully tried to seize Donetsk's international airport on Monday.
Among those killed were 33 Russian nationals, a separatist leader said.
US Secretary of State John Kerry said there was also "evidence of Russians crossing over, trained personnel from Chechnya trained in Russia, who've come across to stir things up, to engage in fighting".
Chechnya's president has denied sending troops to Ukraine.
Separately, US officials said Russia had now withdrawn most of its forces from the border with Ukraine, but thousands remained.
Pro-Russian separatists in the regions of Donetsk and Luhansk declared independence after referendums on 11 May, which were not recognised by Kiev or its Western allies.
The rebels took their cue from a disputed referendum in Crimea, which led to Russia's annexation of Ukraine's southern peninsula.
Ukraine's deadliest clashes
24 April: Ukraine military kills five rebels in assault on Sloviansk
2 May: More than 40 people killed in burning building after street fighting in Odessa
9 May: 20 pro-Russian activists and Ukrainian security officer killed in Mariupol
13 May: Seven Ukrainian soldiers killed in ambush between Sloviansk and Kramatorsk
22 May: Rebel attack on checkpoint in Volnovakha leaves at least 14 soldiers dead
27 May: At least 40 rebels killed in "anti-terrorist operation" against separatists holding Donetsk airport
29 May: Ukrainian military helicopter shot down near Sloviansk, killing 14