By Tom Balmforth and Marko Djurica KYIV/POKROVSK, Ukraine (Reuters) -Russian forces fully occupied the eastern Ukrainian city of Sievierodonetsk on Saturday, both sides said, confirming Kyiv’s biggest battlefield setback for more than a month following weeks of some of the war’s bloodiest fighting. Ukraine called its retreat from the city a “tactical withdrawal” to fight from higher ground in Lysychansk on the opposite bank of the Siverskyi Donets river. Pro-Russian separatists said Moscow’s forces were now attacking Lysychansk. The fall of Sievierodonetsk – once home to more than 100,000 peop…