WASHINGTON (AFP) - An American doctor being treated for Ebola in Nebraska was given a blood transfusion from another American doctor who recovered from the virus, hospital officials said.

Dr Rick Sacra, 51, a Christian missionary doctor, was infected with Ebola while working as an obstetrician in Liberia, and was flown to the United States for treatment last week.

He has been given plasma from Dr Kent Brantly, 33, another US doctor who was infected over the summer with Ebola while treating patients in the Liberian capital, Monrovia. Dr Brantly recovered after being cared for in an Atlanta hospital.

Dr Sacra has been upgraded from serious to good condition, after a week of hospital care, plasma and an experimental treatment doctors declined to name.

Dr Phil Smith, director of the Nebraska Medical Centre biocontainment unit, said it is unclear which treatment has helped Dr Sacra improve.

"We decided we were more interested in saving Rick than trying to do a pure study so we just administered everything we had access to, basically," Dr Smith told reporters on on Thursday.

When Dr Brantly was sickened with Ebola in Monrovia, he also received blood from a child who had recovered from the hemorrhagic virus.

Last week, global health experts in Geneva agreed that blood therapies and convalescent serums can be used to fight Ebola immediately, while safety trials begin for potential vaccines.

"There was consensus," Dr Marie-Paule Kieny, assistant director-general of the World Health Organisation told reporters on Sept 5 following a two-day meeting of some 200 health experts in Geneva.

"A blood-derived product can be used now."

Experts think the presence of Ebola antibodies in the blood of recovered patients may help others fight the disease.

The largest-ever outbreak of Ebola fever has now killed more than 2,400 people and infected nearly 5,000 in West Africa, the United Nations health agency said on Friday.

"In the three hardest-hit countries (Liberia, Sierra Leone and Guinea), the number is moving faster than the capacity to manage them," WHO head Margaret Chan told a news conference in Geneva.