The Ebola virus that has killed scores of people in Guinea this year is a new strain - evidence that the disease did not spread there from outbreaks in other African nations, scientists have reported.

Dr. Stephan Gunther of the Bernhard Nocht Institute for Tropical Medicine in Hamburg, Germany, said: "The source of the virus is still not known."

He led an international team of researchers who studied the genetics of the virus and reported results online in the New England Journal of Medicine.

The outbreak has caused panic and killed more than 120 people in West Africa, mostly in Guinea, according to the World Health Organisation.

Ebola causes internal bleeding and organ failure and is fatal in 30% to 90% of cases, depending on the strain. It spreads through direct contact with infected people, and some earlier cases have been linked to certain fruit bats 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 said the Guinea Ebola was a Zaire strain, different from the kind that has caused cases in other parts of Africa. 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, Mr Gunther said.

Researchers think the Guinea and other strains evolved in parallel from a recent ancestor virus. The Guinea outbreak is likely to have started in 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.