ANCHORAGE, Alaska, Jan 14 (Reuters) - Five months after suing to keep polar bears off the U.S. threatened species list, Alaska's government said Wednesday it plans to issue a similar challenge to block federal protections for a struggling population of beluga whales in Cook Inlet, a mature oil-producing basin. Former vice presidential hopeful Gov. Sarah Palin said the energy-rich state believes the Endangered Species Act protections for belugas announced in October by the National Oceanic and Atmospheric Administration are unwarranted.