An experimental Ebola vaccine made by GlaxoSmithKline has caused no serious side effects in an early trial, scientists reported in the New England Journal of Medicine.

The vaccine also produced an immune response in all 20 healthy volunteers who received it.

The World Health Organisation has said 5,689 people have now died in the worst outbreak of the virus.

Almost all of the 15,935 reported cases and all but 15 deaths have been in the west African nations of Guinea, Sierra Leone and Liberia.

The trial, which began on 2 September and will monitor the volunteers for 48 weeks, is primarily aimed at assessing how safe the vaccine is.

But the immune response offered hope that it would also be effective.

"The safety profile is encouraging, as is the finding that the higher dose of vaccine induced an immune response quite comparable to that which has completely protected (lab) animals from Ebola," said Dr Anthony Fauci.

Dr Fauci is director of the National Institute of Allergy and Infectious Diseases (NIAID), which is conducting the trial in Bethesda, Maryland.

The intramuscular vaccine was developed at NIAID and Okairos, a biotechnology company acquired by GlaxoSmithKline.

It contains genetic material from two Ebola strains - Zaire, responsible for the current outbreak in west Africa, and Sudan, but no virus, so it cannot cause the disease.

Because it is unethical to expose volunteers to Ebola, researchers assess the effectiveness of candidate vaccines by whether they trigger production of anti-Ebola antibodies and immune-system T cells.

The trial enrolled volunteers ages 18 to 50. Half received a lower dose and half a higher dose.

All 20 developed anti-Ebola antibodies within four weeks, with those on the higher dose producing more.