Ukrainian forces were holding their positions in intense street fighting and under day and night shelling in Sievierodonetsk, officials said, as Russia pushes to control the bombed-out city, key to its objective of controlling eastern Ukraine. Sievierodonetsk and its twin city Lysychansk, on the opposite bank of the Siverskyi Donets river, are the last Ukrainian-controlled parts of Luhansk province, which Russia is determined to seize as one of its principal war objectives. Ukraine’s Security Council Secretary Oleksiy Danilov said on Thursday the situation in Sievierodonetsk was “extremely com…