WASHINGTON (AFP) - US health authorities on Thursday issued a travel warning for three African nations struggling with the largest outbreak of deadly Ebola virus in history.

The Centres for Disease Control and Prevention "recommends against non-essential travel to Guinea, Liberia and Sierra Leone," said director Tom Frieden.

"This is because the ongoing Ebola outbreak represents a potential risk to travelers," he said.

The CDC is also sending 50 extra specialists to the affected areas in West Africa, he said.

The current outbreak of Ebola has killed 729 people in Sierra Leone, Guinea, Nigeria and Liberia, the World Health Organisation said.

The virus can be fatal in up to 90 per cent of cases, though this outbreak has killed about 60 per cent of those infected.