(CNN) -- The Ebola outbreak in coastal West Africa is still contained to Guinea and Liberia, the World Health Organization announced Tuesday, despite rumors of the virus spreading to other countries.
Cases have been reported in Sierra Leone, Mali and Ghana, but the WHO says none has been confirmed. Rumored cases in Mali are still being investigated.
The number of suspected cases in Guinea has grown to 157, with 101 deaths. Sixty-seven have been confirmed as Ebola. In Liberia, 21 cases have been reported, including 10 deaths. Five of the cases have been confirmed as Ebola.
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.
Twenty cases are believed to have occurred in Guinea's capital, Conakry, according to WHO.
"This is one of the most challenging Ebola outbreaks that we have ever faced," said Dr. Keiji Fukuda, WHO's assistant director-general for health security.
It's the first emergence of Ebola in western Africa, which means doctors and health officials in the region don't have any experience with the virus. Outbreaks generate a lot of fear and anxiety, Fukuda said, which can lead to misinformation.
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," spokesman Tarik Jasarevic told CNN last week.
There are about 50 staff currently deployed in the area and more are set to go shortly. They are working to prevent the spread of infection, primarily in health care facilities, and are sending personal protective equipment to surrounding countries.
"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," Jasarevic said. "They need to know what it is and how they can protect themselves."
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 last week that biological samples tied to three suspected Ebola cases within its borders were being sent to the U.S. Centers for Disease Control and Prevention for analysis. WHO said two of the samples have come back negative; they are still waiting on the other. Nine Ebola cases total are suspected in the country.
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.
Close to 9 in 10 of the patients WHO health professionals are seeing with Ebola are dying, said Dr, Stephane Hugonnet, who returned from southeast Guinea last weekend.
"Ebola is clearly a severe disease," Fukuda said. "It is also an infection that can be controlled. ... We know very well how this virus is transmitted."
The virus is named after the Ebola River in the Democratic Republic of Congo (formerly Zaire), where one of the first recorded outbreaks occurred in 1976.
CNN's Miriam Falco, Jacque Wilson, Matt Smith and Anna Maja Rappard contributed to this report.