World Health Organization says "one of most challenging" Ebola outbreaks ever is spreading in West Africa.

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."

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.

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.