By Pavel Polityuk, Tom Balmforth and Terje Solsvik KYIV/OSLO (Reuters) -Russia intensified an offensive in the Donbas region of eastern Ukraine on Saturday and stopped providing gas to Finland, escalating Moscow’s dispute with the West over energy payments. After ending weeks of resistance by the last Ukrainian fighters in the strategic southeastern city of Mariupol, Russia is waging what appears to be a major offensive in Luhansk, one of two provinces in Donbas. Russian-backed separatists already controlled swathes of territory in Luhansk and the neighbouring Donetsk province before the Feb. …