By Tom Balmforth, Pavel Polityuk and Terje Solsvik KYIV/OSLO (Reuters) -Ukraine ruled out a ceasefire or concessions to Moscow on Saturday as Russia intensified an offensive in the eastern Donbas region and stopped providing gas to Finland. 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. 24 invasion, but …