By Pavel Polityuk and Conor Humphries KYIV/SVITLODARSK, Ukraine (Reuters) -Advancing Russian forces came closer to surrounding Ukrainian troops in the east, briefly seizing positions on the last highway out of a crucial pair of Ukrainian-held cities before being beaten back, a Ukrainian official said on Thursday. Three months into its invasion of Ukraine, Russia has abandoned its assault on the capital Kyiv and is trying to consolidate control of the industrial eastern Donbas region, where it has backed a separatist revolt since 2014. It has poured thousands of troops into its assault, attacki…