The World Health Organization (WHO) on Thursday called for "drastic action" to fight the deadliest Ebola outbreak on record, as it announced an 11-nation meeting to address the growing crisis.

The outbreak that began in Guinea has turned into a cross-border crisis that could spread to more countries, the WHO said.

Despite efforts by national health authorities and international aid organizations to contain its spread, the WHO has recorded 635 infections, including 399 deaths, in Guinea, Sierra Leone and Liberia since the outbreak began in February. The number of infections continues to rise.

The crisis is already the deadliest outbreak since Ebola first emerged in central Africa in 1976 — when 318 people were infected and 280 killed in what was then Zaire.

"This is no longer a country specific outbreak but a sub-regional crisis that requires firm action by governments and partners," Luis Sambo, WHO Regional Director for Africa, said in a statement.

"WHO is gravely concerned by the on-going cross-border transmission into neighboring countries as well as the potential for further international spread," he said.