LONDON (Reuters) – A Russian-backed separatist leader said on Wednesday that 140,000 people had left the Ukrainian city of Mariupol for Russia or the breakaway Donetsk People’s Republic since Russian forces began besieging it, the Interfax news agency reported. The port city of Mariupol, which had a prewar population of more than 400,000, is a key target for Russia in its apparent attempt to create a land bridge between the Crimean peninsula, which Russia seized from Ukraine in 2014, and pro-Russian separatist regions in eastern Ukraine. Denis Pushilin, head of the self-styled Donetsk People’s…