By Carl Odera and Michelle Nichols JUBA/UNITED NATIONS (Reuters) - The U.N. Security Council approved plans on Tuesday to almost double the number of peacekeepers in South Sudan in an effort to protect civilians as reports of mass graves fueled fears of a worsening of ethnic bloodletting in the world's newest state. Dau said production in Upper Nile state, where most of South Sudan's oil is extracted, was safe and outside the reach of rebels.