Fighting has intensified around Sloviansk and 12 Ukrainian troops were killed on Thursday when pro-Russian rebels shot down an army helicopter.Dozens of separatists died on Monday in a battle at Donetsk airport.Russia has again called for Ukraine to stop its military campaign against the pro-Moscow rebels.Interim Defence Minister Mykhailo Koval told reporters Ukrainian forces would continue operations in border areas "until these regions begin to live normally, until there is peace," the Reuters news agency reported.And he again accused Russia of backing the rebels - a claim Moscow denies.President-elect Petro Poroshenko has said Ukraine will punish the "bandits" who shot down the helicopter. Among those killed was Maj Gen Serhiy Kulchytskiy, head of combat and special training in Ukraine's National Guard.The leader of the self-declared Donetsk People's Republic, Alexander Borodai, told Russia's TV Rain that the only condition for negotiations with Kiev was the withdrawal of its troops from the territories of Donetsk and Luhansk.And he denied any knowledge of the whereabouts of a team of monitors from the Organization for Security and Co-operation in Europe (OSCE), held by separatists. The self-proclaimed mayor of Sloviansk has said they will be freed soon but nothing has been heard of them since Monday.A second group of observers, based in Luhansk, remains unaccounted for.Separatists there told the Interfax-Ukraine news agency they had released the team, but the OSCE has put out a statement saying it lost contact with the observers at 19:00 local time (16:00 GMT) on Thursday.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.Up to 33 Russian nationals were among the dozens of rebels killed in fighting around Donetsk airport on Monday.US Secretary of State John Kerry said there was "evidence" of Chechens trained in Russia who had come to "stir things up".Chechnya's president has denied sending troops to Ukraine.Separately, US officials said Russia had now withdrawn most of its forces from the border with Ukraine, but thousands remained.Ukraine is also under pressure from Russia over its unpaid gas bill, which runs into billions of dollars.Russia's state gas company Gazprom has threatened to cut off supplies to Ukraine if it fails to pay in advance for its June supplies.A new round of talks between Ukraine, Russia and the EU is due to take place in Berlin later on Friday.(BBC)Bakudaily.az