A protester holds a placard during a demonstration against the killing of Yazidi's by Islamic State fighters in northern Iraq, outside Downing Street in central London on August 13, 2014. An international plan is under way to rescue civilians trapped by Islamic State (IS) fighters on a mountain in northern Iraq, British Prime Minister David Cameron said Wednesday. Cameron declined to give details of the operation but said Britain would play a role, just as it had worked alongside the United States in conducting humanitarian aid drops to thousands of Yazidis and other minorities who have fled to Mount Sinjar. AFP PHOTO / JUSTIN TALLIS (Photo credit should read JUSTIN TALLIS/AFP/Getty Images)

A protester holds a placard during a demonstration against the killing of Yazidi's by Islamic State fighters in northern Iraq, outside Downing Street in central London on August 13, 2014. An international plan is under way to rescue civilians trapped by Islamic State (IS) fighters on a mountain in northern Iraq, British Prime Minister David Cameron said Wednesday. Cameron declined to give details of the operation but said Britain would play a role, just as it had worked alongside the United States in conducting humanitarian aid drops to thousands of Yazidis and other minorities who have fled to Mount Sinjar. AFP PHOTO / JUSTIN TALLIS        (Photo credit should read JUSTIN TALLIS/AFP/Getty Images)