Ebola causes internal bleeding and organ failure and is fatal in 30 per cent to 90 per cent of cases, depending on the kind of strain. It spreads through direct contact with infected people, and some earlier cases have been linked to certain fruit bats that live in West Africa.

There is no cure or vaccine, so containing the outbreak has focused on supportive care for those infected with the virus and isolating them to limit its spread.

Earlier, health officials had said the Guinea Ebola was a Zaire strain that was different from the kind that has caused cases in other parts of Africa.

The Democratic Republic of Congo used to be called Zaire.

The new research analysed blood samples from 20 patients in the current outbreak and found the strain was unique.

"It is not coming from the Democratic Republic of Congo. It has not been imported to Guinea" from that country or from Gabon, where Ebola has also occurred, Gunther said.

Researchers think the Guinea and other strains evolved in parallel from a recent ancestor virus. The Guinea outbreak likely began last December or earlier and might have been smouldering for some time unrecognised. The investigation continues to try to identify "the presumed animal source", they write.

The outbreak in Guinea is one of the deadliest in history, with 168 cases "clinically compatible" with Ebola virus disease reported, including 108 deaths, since the start of the year, according to the World Health Organisation.

The outbreak began in the impoverished country's southern forests but has spread to Conakry, a sprawling port city on the Atlantic coast.

Neighbouring Liberia has reported 20 probable or suspected cases, six lab-confirmed cases and 13 deaths.

Mali also had suspected cases but was given the all-clear on Tuesday after samples taken from patients tested negative for Ebola in laboratories, the health ministry said.