Aid organisation Doctors Without Borders said Monday an Ebola outbreak suspected of killing dozens in Guinea was an "unprecedented epidemic" as Liberia confirmed its first cases of the deadly contagion.

Guinea's health ministry this year has reported 122 "suspicious cases" of viral haemorrhagic fever, including 78 deaths, with 22 of the samples taken from patients testing positive for the highly contagious tropical pathogen.

"We are facing an epidemic of a magnitude never before seen in terms of the distribution of cases in the country: Gueckedou, Macenta, Kissidougou, Nzerekore, and now Conakry," Mariano Lugli, the organisation's coordinator in the Guinean capital, said in a statement.

The group, known by its French initials MSF, said that by the end of the week it would have around 60 international field workers with experience in working on haemorrhagic fever divided between Conakry and the south-east of the country.

"MSF has intervened in almost all reported Ebola outbreaks in recent years, but they were much more geographically contained and involved more remote locations," Lugli said.

"This geographical spread is worrisome because it will greatly complicate the tasks of the organisations working to control the epidemic."

The World Health Organization (WHO) and local health authorities have announced two Ebola cases among seven samples tested from Liberia's northern Foya district, confirming for the first time the spread of the virus across international borders.

Liberian Health Minister Walter Gwenigale told reporters the patients were sisters, one of whom had died.

The surviving sister returned to Monrovia in a taxi before she could be isolated and the authorities fear she may have spread the virus to her taxi driver and four members of her family.

The woman and those with whom she has come into contact are in quarantine in a hospital 48 kilometres (30 miles) south-east of Monrovia, Gwenigale said.

-- Unstoppable bleeding --

Ebola has killed almost 1,600 people since it was first observed in 1976 in what is now the Democratic Republic of Congo but this is the first fatal outbreak in west Africa.

The tropical virus leads to haemorrhagic fever, causing muscle pain, weakness, vomiting, diarrhoea and, in severe cases, organ failure and unstoppable bleeding.

The WHO said Sierra Leone has also identified two suspected cases, both of whom died, but neither has been confirmed to be Ebola.

No treatment or vaccine is available for the bug, and the Zaire strain detected in Guinea has a historic death rate of up to 90 percent.

It can be transmitted to humans from wild animals, and between humans through direct contact with another's blood, faeces or sweat, as well as sexual contact or the unprotected handling of contaminated corpses.

MSF said it had stepped up support for the isolation of patients in Conakry, in collaboration with the Guinean health authorities and the WHO.

"Other patients in other health structures are still hospitalised in non-optimal conditions and isolation must be reinforced in the coming days," it added.

The WHO said it was not recommending travel or trade restrictions to Liberia, Guinea or Sierra Leone based on the current information available about the outbreak.

But Senegal has closed border crossings to Guinea "until further notice".