Pakistani authorities have begun evacuating more than half a million people from flood-risk areas with the overall number of people affected by flood waters topping four million. The floods have ravaged north-west of the country killing more than 1500 in the past week, washing away roads, bridges and homes and destroying crops and farmland.Chief of the UN Office for Coordination of Humanitarian Assistance in Pakistan, Manuel Bessler says, the floods have washed roads away and destroyed bridges leaving many areas accessible only by helicopters.