THE West African Ebola outbreak is accelerating and could grow six times larger and infect as many as 20,000 people, the World Health Organisation warned on Thursday.

The UN health agency unveiled a new road map for containing the virus and said scientists are fast-tracking efforts to find a treatment or vaccine.

An experimental vaccine developed by the US government and GlaxoSmithKline will be tested on humans starting next week, the US National Institutes of Health announced.

The NIH trial will use healthy adult volunteers in Maryland and British experts will simultaneously test the same vaccine in healthy people in Britain, Gambia and Mali.

Preliminary results on the vaccine’s safety — not its effectiveness — would not be available for months.

The latest outbreak has killed at least 1,552 people of the 3,069 reported cases in Liberia, Sierra Leone, Guinea and Nigeria and the real caseload in urban areas could be two to four times higher.

But the WHO announcement was immediately questioned by medical charity Doctors Without Borders (MSF), which runs many of the treatment centres in west Africa.

“The WHO road map is welcome, but it should not give a false sense of hope,” said MSF operations director Bruce de le Vingne.

“A plan needs to be acted upon. Huge questions remain.

“States with the capacity to help have the responsibility to mobilise resources to the affected countries, rather than watching from the sidelines with a naive hope that the situation will improve.”