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.
"Today, our boys died tragically," Poroshenko said on Thursday.
"This is the moment of deep sorrow, and I express my sincere condolences to the families and relatives of the victims.
"We must make every effort to make sure that no more Ukrainians die at the hands of terrorists and bandits."
Poroshenko, who won last Sunday's presidential elections, earlier pledged to tackle separatists "in hours", not months.
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.
Six National Guard soldiers - including Maj Gen Serhiy Kulchytskiy, head of combat and special training - and six members of the special forces of the interior died, the National Guard said in a statement.
It added that the "criminals" who attacked the helicopter were later "destroyed" by Ukrainian troops involved in an "anti-terror" operation.
It is one of the worst losses of life for government forces in the conflict. Last week at least 14 soldiers died in a rebel attack on an army checkpoint near Donetsk, some 130km (80 miles) from Sloviansk.
Earlier this month, separatists shot down two army helicopters, also near Sloviansk, killing a pilot and another serviceman.
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.
Alexander Borodai, the separatist leader of the self-proclaimed Donetsk People's Republic, said 33 Russian nationals had been among those killed in the airport clashes.
Borodai, himself a Russian citizen, added that their bodies had been identified and would be taken to Russia.
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.