Guinea has received confirmation that a mysterious disease that has killed up to 59 people in the West African country is the haemorrhagic fever Ebola.
It may also have spread to neighbouring Sierra Leone.
It is among the most virulent pathogens known to infect humans, with a fatality rate of up to 90%.
Cases of the disease have been registered in three southeastern towns and in the capital Conakry since 9 February.
It has never before been recorded in Guinea.
Six of the 12 samples sent for analysis tested positive for Ebola, said Dr Sakoba Keita, who heads the epidemics prevention division at Guinea's health ministry.
He said that health officials had registered 80 suspected cases of the disease, including 59 deaths.
"But you have to understand that not all the cases are necessarily due to Ebola fever. Some will have other origins, including a form of severe dysentery," Mr Keita said.
World Health Organisation (WHO) officials said that cases showing similar symptoms, including fever, diarrhoea, vomiting and bleeding, had also been reported in an area of Sierra Leone near the border with Guinea.
Sierra Leone's chief medical officer, Dr Brima Kargbo, said authorities were investigating the case of a 14-year-old boy who died in the town of Buedu in the eastern Kailahun District.
The boy had travelled to Guinea to attend the funeral of one of the outbreak's earlier victims.
Mr Kargbo said a medical team had been sent to Buedu to test those who came into contact with the boy before his death.
The international medical charity Medecins Sans Frontieres (MSF) announced on Saturday it was reinforcing its medical and logistics teams in Guinea in response to the epidemic.
It is also flying in 33 tonnes of medicines and equipment and is setting up isolation units in the three affected towns in Guinea.
"These structures are essential to prevent the spread of the disease, which is highly contagious," Dr. Esther Sterk, MSF's tropical medicine adviser, said in a statement. "Specialised staff are providing care to patients showing signs of infection."
Ebola is introduced into the human population through close contact with infected animals including chimpanzees, gorillas, fruit bats, monkeys, forest antelope and porcupines, according to the WHO.
The disease, which is transmitted between humans through contact with organs, blood, secretions, or other bodily fluids, is most commonly found in the Democratic Republic of Congo, Uganda, South Sudan and Gabon.
Though no epidemics of the disease have been recorded among humans in West Africa, a variety of Ebola infected a colony of chimpanzees in Ivory Coast's Tai National Park, near the country's border with Liberia, in 1994.
A Swiss scientist, who performed an autopsy on one of the infected animals, contracted the disease but later recovered.