(CNN) -- The death toll from an Ebola outbreak in coastal West Africa has risen to 86, with dozens more ill, aid workers reported Friday.
The deaths are among the 137 cases reported by the World Health Organization, which said the outbreak has "rapidly evolved" since originating in the forests of southeastern Guinea. The city of Guekedou, near the borders with Sierra Leone and Liberia, has seen the majority of the deaths.
Five people are believed to have died in Guinea's capital, Conakry, according to WHO. Two of the victims had traveled to the region.
It's the first emergence of Ebola in western Africa, and WHO spokesman Tarik Jasarevic said the U.N. agency is trying to track people who had encountered the victims and make sure "that all those who have been in contact with infected people are being checked upon."
"What is really important is to inform the population of Guinea and Conakry about this disease, as this is the first time they are facing Ebola. They need to know what it is and how they can protect themselves."
In Liberia, seven Ebola deaths have been confirmed out of 14 suspected cases. Sierra Leone is investigating at least two deaths. The aid organization Doctors Without Borders has called the outbreak unprecedented, because previous cases have been limited to a small area.
Mali's government reported on its Facebook page on Thursday that biological samples tied to three suspected Ebola cases within its borders are being sent to the U.S. Centers for Disease Control and Prevention for analysis.
In the meantime, the three people there suspected of having the disease are being treated in an isolated unit, where their health is improving.
Ebola is one of the world's deadliest viruses, causing a hemorrhagic fever that kills up to 90% of those infected. It spreads in the blood and shuts down the immune system, causing high fever, headache and muscle pain, often accompanied by bleeding.
The virus is named after the Ebola River in the Democratic Republic of Congo (formerly Zaire), where one of the first outbreaks occurred in 1976.
CNN's Anna Maja Rappard contributed to this report.