(Reuters) – Ukrainian troops were set to withdraw from the eastern city of Sievierodonetsk after weeks of intense bombardments and street fighting, the regional governor said on Friday. The day marks four months since Russian President Vladimir Putin sent forces across the border into Ukraine, sparking the biggest conflict in Europe since World War Two. FIGHTING * Luhansk governor Serhiy Gaidai said Ukrainian forces will have to leave Sievierodonetsk. Troops had already received the order to move to new positions, he added. * Russian forces are seeking to surround eastern Ukraine’s embattled L…