(CNN) -- The Ebola outbreak is much worse than official figures show and is "spiraling out of control," a leading U.S. official said Tuesday -- due in part, he said, to some countries that inadvertently have made it harder to corral the deadly disease. Centers for Disease Control and Prevention Director Dr. Tom Frieden offered his stark commentary to CNN a day after Ellen Johnson Sirleaf, the president of Liberia, voiced dissatisfaction with the world response so far. "In a way, we feel saddened by the response," President Sirleaf said. More than 3,000 people have been infected by Ebola in Liberia, Guinea, Sierra Leone and Nigeria since the first documented cases in December, according to the World Health Organization. At least 1,552 have died. Human trial of experimental Ebola vaccine begins .
NEW: Obama to West Africans: "We know how to" stop Ebola from spreading . NEW: U.S. official: 500 from CDC working on Ebola; 100 government workers in region . CDC director: "Hugely fast increase" in harder-to-manage Ebola cases of late . Liberian President: "We feel saddened" by the international response to the crisis .