An Ebola outbreak continues to spread in three West African countries, and the death toll in the outbreak has risen to 337, the World Health Organization says.

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 UN health agency, wrote in an email.

"This makes this one of the most challenging Ebola outbreaks ever."

500 suspected cases

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

That appears to be a large increase 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 organisation said the numbers were constantly in flux as test results came 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 was required to get sick people into treatment facilities and to track down people that the sick had come into contact with, so they could be monitored for symptoms.