DAKAR, Senegal (AP) — An Ebola outbreak continues to spread in three West African countries, and the death toll in the outbreak has risen to more than 330, the World Health Organization said Wednesday.

Health officials have struggled to contain the outbreak, which is believed to have begun in Guinea, where the majority of the cases and deaths have been. It has also touched Sierra Leone and Liberia, where it recently flared again after about two months with no new cases.

"This is a complex outbreak involving multiple locations in three countries with a lot of cross-border movement among the communities," Fadela Chaib, a spokeswoman for the U.N. health agency, wrote in an email. "This makes this one of the most challenging Ebola outbreaks ever."

In an update published on its website Wednesday, the agency said that more than 500 suspected or confirmed cases of the virus have been recorded. It said 337 people have died.

That appears to be a large uptick since the last update, published about a week earlier, when the agency reported about 240 had died of the disease. But there is sometimes a significant lag in tallying cases, and the organization said the numbers are constantly in flux as tests come in.

"The jump in cases is due to reclassification, retrospective investigation, and consolidation of cases," Chaib wrote.

This is the first time Ebola has struck three countries at once and the first major outbreak in West Africa. Fear of the disease, for which there is no cure, has hampered efforts to isolate the sick.

Chaib said more work needs to be down to get sick people into treatment facilities and to track down people that the sick have come into contact with, so they can be monitored for symptoms.