By David Ljunggren and Lidia Kelly (Reuters) – Russia’s defence ministry has told the Ukrainian forces still fighting in the besieged southern port of Mariupol to lay down their arms starting 6 a.m. Moscow time (0300 GMT) on Sunday to save their lives. Russia’s earlier claim that its troops had cleared the urban area of Mariupol, scene of the war’s heaviest fighting and worst humanitarian catastrophe, could not be independently verified. It would be the first major city to have fallen to Russian forces since the Feb. 24 invasion. Moscow said the remaining fighters in Mariupol – who it says are…