By Max Hunder and Conor Humphries KYIV (Reuters) -Russian forces stepped up their assault on the Ukrainian city of Sievierodonetsk on Saturday after claiming to have captured the nearby rail hub of Lyman, as Kyiv intensified its calls for longer-range weaponry from the West to help it fight back in the Donbas region. Slow, solid Russian gains in recent days point to a subtle momentum shift in the war, now in its fourth month. The invading forces appear close to seizing all of the Luhansk region of Donbas, one of the more modest war goals the Kremlin set after abandoning its assault on Kyiv in …