By Pavel Polityuk and Jonathan Landay KYIV/VILKHIVKA, Ukraine (Reuters) -Ukrainian forces reported battlefield gains on Wednesday in a counterattack that could signal a shift in the momentum of the war, while Kyiv shut gas flows on a route through Russian-held territory, raising the spectre of an energy crisis in Europe. Following days of advances north and east of the second largest city Kharkiv, Ukrainian forces were within just several kilometres of the Russian border on Wednesday morning, one Ukrainian military source said on condition on anonymity. Before the advance, Russian forces had b…