The Ebola virus has been identified as the cause of an outbreak of haemorrhagic fever now believed to have killed nearly 60 people in southern Guinea, government officials say.
Scores of cases have been recorded since the outbreak began early last month.
There is no known cure or vaccine for the highly contagious Ebola virus.
It is spread by close personal contact with people who are infected and kills between 25% and 90% of victims.
Symptoms include internal and external bleeding, diarrhoea and vomiting.
Outbreaks of Ebola occur primarily in remote villages in Central and West Africa, near tropical rainforests, the World Health Organization says.
Analysts suggest it has never been recorded in Guinea before.
Recent years have seen outbreaks in Uganda and Democratic Republic of Congo.
“We got the first results from Lyon yesterday (Friday) which informed us of the presence of the Ebola virus as the cause of this outbreak,” Guinean health ministry official Sakoba Keita told AFP.
“The Ebola fever epidemic raging in southern Guinea since 9 February has left at least 59 dead out of 80 cases identified by our services on the ground.”
“We are overwhelmed in the field, we are fighting against this epidemic with all the means we have at our disposal with the help of our partners but it is difficult.”
Medical aid charity Medecins sans Frontieres said on Saturday it would strengthen its team in Guinea and fly some 33 tonnes of drugs and isolation equipment in from Belgium and France.
Dr Armand Sprecher, an emergency physician and epidemiologist working with MSF in Guinea, told the BBC that doctors had to identify all patients with the disease and monitor anyone they had been in contact with during their illness.
The latest outbreak could be brought under control if people acted quickly, he said.
“Based on our history with these sorts of outbreaks it will happen. Ideally, sooner rather than later,” said Dr Sprecher.
“The more quickly we can contain this fewer cases we’ll have, then the smaller the scale of the epidemic. That’s the idea of going in as strong as we can early on.”
Other reports claim that there is the likelihood that the virus has already reached neighboring Sierra Leone.
Meanwhile, some Liberians who have heard the news have expressed serious concern, and have therefore called on the Government of Liberia to be proactive in taking preventive measures.