DOLO, Somalia — Is the world about to watch 750,000 Somalis starve to death? The United Nations’ warnings could not be clearer. A drought-induced famine is steadily creeping across Somalia and tens of thousands of people have already died. The Islamist militant group the Shabab is blocking most aid agencies from accessing the areas it controls, and in the next few months three-quarters of a million people could run out of food, United Nations officials say.