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World leaders are meeting in Istanbul for two days to come up with a more coherent response to what U.N. chief Ban Ki-moon has called the worst humanitarian situation since World War Two. More than 130 million people are affected by war or natural disaster around the world. Our correspondent, Mark Lowen, who's at the summit, explains what issues they'll be addressing.

(Image: The Turkish President Recep Tayyip Erdogan (R) and UN Secretary General Ban Ki-moon (L) attend the UN Humanitarian Summit in Istanbul, Turkey. Credit: EPA)