At least two separatists were reported killed as commandos, backed by armoured vehicles, cleared checkpoints on the outskirts of the town.But a BBC correspondent says the centre of Sloviansk is currently quiet.Pro-Russian separatists are occupying key buildings in at least a dozen eastern towns, defying Kiev.There was also further unrest earlier on Thursday in the south-eastern city of Mariupol.Interior Minister Arsen Avakov said government forces had ousted pro-Russian militants from the city hall building. But another BBC correspondent says there is no sign of Ukrainian troops anywhere nearby.Speaking on Russian TV as news of the Sloviansk operation was coming in, Russian President Vladimir Putin said: "If the regime in Kiev has begun using the army against the population inside the country, then this is undoubtedly a very serious crime.""Of course, this will have consequences for the people who take such decisions, and this also affects our inter-state relations," he added.In another development, the funeral is taking place for a pro-Ukrainian politician, Volodymyr Rybak, who was abducted and killed in Donetsk region.Meanwhile, US President Barack Obama has accused Russia of flouting a deal on Ukraine reached in Geneva last week, under which illegal armed groups, including those who have seized public buildings, would return home.Mr Obama told a news conference in Japan that Moscow had failed to halt actions by militants in the region and warned that the US had further sanctions against Russia "teed up".A contingent of US troops has begun landing in Poland for military exercises amid concerns among Nato's eastern member states about Russia's intentions.Moscow has said it will respond to any attack on its interests in Ukraine.Unrest began in Ukraine last November over whether the country should look towards Moscow or the West.(BBC)ANN.Az