By Natalia Zinets and Conor Humphries KYIV/POPASNA, Ukraine (Reuters) -Russian forces in eastern Ukraine captured the centre of the railway hub town of Lyman and encircled most of Sievierodonetsk city, Ukrainian officials said on Friday, as Kyiv’s forces fell back in the face of Moscow’s biggest advance for weeks. Ukraine said its forces still held new defensive lines in the eastern Donbas region, despite apparent Russian advances on two major fronts there that showed how momentum has shifted in recent days. Moscow’s separatist proxies said they were in full control of Lyman, which Russia has …