By Natalia Zinets KYIV (Reuters) -Russia warned Lithuania on Tuesday that it would face measures of a “serious negative impact” for blocking some shipments by rail to Moscow’s Baltic Sea enclave of Kaliningrad, in its latest dispute over Western sanctions imposed on the country for the war in Ukraine. Russian forces and separatists in eastern Ukraine made further advances, pushing towards the city of Lysychansk, the Ukrainian forces’ main bastion in an area that is part of the Donbas region Moscow claims for the separatists. Ukraine, its forces and weaponry dwarfed by Russia’s, continues to as…