The U.N. World Health Organization (WHO) warned Thursday that dramatic measures were necessary to combat a deadly Ebola outbreak in West Africa, calling a meeting of health ministers from 11 countries to address the crisis. "As the number of deaths and cases of Ebola virus continues to rise in Guinea, Liberia, and Sierra Leone, the World Health Organization is warning that drastic action is needed," the agency said in a statement from Geneva. Despite efforts by national health authorities and international aid organizations to contain its spread, the WHO has recorded 635 infections, including 399 deaths in the three countries since the outbreak began in February. The crisis already is the deadliest outbreak since Ebola first emerged in central Africa in 1976, and the number of infections continues to rise. "This is no longer a country-specific outbreak, but a sub-regional crisis that requires firm action by governments and partners," Luis Sambo, the WHO regional director for Africa, said in a statement. "WHO is gravely concerned by the ongoing cross-border transmission into neighboring countries as well as the potential for further international spread." In response to the crisis, the WHO said it will convene the health-ministers meeting in Accra, Ghana on July 2 and 3 to develop a comprehensive inter-country response plan. Ebola—with a fatality rate of up to 90 percent, no vaccine, and no known cure—has not occurred previously in West Africa. The virus causes high fever and other symptoms before moving into more severe phases including internal and external hemorrhaging.