"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.The helicopter arrived at Karachun [near Sloviansk], to unload and pick up a group of people who were finishing their tour and going on leave. They were shot down after take-off. I had talked to the pilot and also to Gen Kulchytskiy. The pilots were very nice guys - polite, correct, friendly.Gen Kulchytskiy was very capable and very accessible. A good commander who would fly to the checkpoints where his soldiers were. He would personally bring them food and water. He kept an eye on everything.The fighting here has become more frequent recently. In the last few days, it broke out even during the daytime. Previously, it happened only at night.However, the last two days were relatively quiet. The army man checkpoints, preventing the forces of the People's Republic of Donetsk from moving about freely. The mood is good among the soldiers. No desertion or talk about giving up. People are in good fighting spirit.'Criminals destroyed'"Today, our boys died tragically," Mr 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.Helicopter crash scene"We must make every effort to make sure that no more Ukrainians die at the hands of terrorists and bandits."Mr 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.'Independence referendums'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.Mr 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.lineUkraine's deadliest clashes24 April: Ukraine military kills five rebels in assault on Sloviansk2 May: More than 40 people killed in burning building after street fighting in Odessa9 May: 20 pro-Russian activists and Ukrainian security officer killed in Mariupol13 May: Seven Ukrainian soldiers killed in ambush between Sloviansk and Kramatorsk22 May: Rebel attack on checkpoint in Volnovakha leaves at least 14 soldiers dead27 May: At least 40 rebels killed in "anti-terrorist operation" against separatists holding Donetsk airport29 May: Ukrainian military helicopter shot down near Sloviansk, killing 14(BBC)Bakudaily.az