The World Health Organization is warning that an Ebola virus outbreak in Congo is very serious and could spread to larger centres.

"If nothing is done now, the disease will reach other places, and even major towns will be threatened," WHO spokesman Eugene Kabambi told Reuters in Kinshasa, the capital of Congo.

Reuters said 31 people have died, including five health-care workers.

On its Twitter feed, WHO said the situation was "not out of control as reported by many."

The outbreak is currently limited to Isiro and Viadana, two villages in a northeastern province of Congo.

WHO says there are about 65 probable or suspected cases of Ebola in the country. Another 108 people were reported to be under surveillance.

Ebola, which causes massive bleeding, kills up to 90 per cent of the people who contract it, according to WHO. Congo has been the scene of several outbreaks of the disease.

The virus is transmitted by direct contact with the blood, body fluids and tissues of infected persons.

Transmission of the virus has also occurred by handling sick or dead infected wild animals including chimpanzees, gorillas, monkeys, forest antelope and fruit bats.