San Diego - One of only six northern white rhino left on earth died over the weekend at a San Diego Zoo, bringing the species closer to extinction, zoo officials said on Monday.Angalifu, who was about 44 years old and being treated for age-related conditions, died on Sunday, according to the San Diego Zoo Safari Park.The world's only known remaining northern white rhino are an elderly female still at the Safari Park, one at a zoo in Europe and three in Africa, the California Zoo said in a statement."Angalifu's death is a tremendous loss to all of us," Randy Rieches, curator of mammals for the zoo, said in a statement. "Not only because he was well beloved here at the Park, but also because his death brings this wonderful species one step closer to extinction."Northern white rhino have been nearly eliminated because of poaching in Africa, the zoo said. Only a few have been preserved at zoos and largely they have not re-produced.In the wild, rhino are killed for their horns, which some people believe have medicinal value.Semen and testicular tissue from the male rhino have been stored with the hope that new reproductive technologies will allow recovery of the species, the zoo said.