The death toll from an Ebola outbreak in West Africa has risen to 603 since February, with at least 68 deaths reported from three countries in the region in the last week alone, the World Health Organisation (WHO) has said.

WHO said on Tuesday that there had been 85 new cases between July 8 to July 12, highlighting continued high levels of transmission.

International and local medics were struggling to get access to communities as many residents feared outsiders were spreading rather than fighting Ebola.

"It's very difficult for us to get into communities where there is hostility to outsiders," WHO spokesman Dan Epstein told a news briefing in Geneva.

"We still face rumours, and suspicion and hostility. People are isolated, they're afraid, they're scared."

Sierra Leone recorded the highest number of deaths, which include confirmed, probable and suspected cases of Ebola, with 52, the Reuters news agency reported.

Liberia reported 13 and Guinea three, according to the WHO figures.

Epstein said the main focus in the three countries is tracing people who have been exposed to people with Ebola and monitoring them for the 21-day incubation period to see if they were infected.

"It's probably going to be several months before we are able to get a grip on this epidemic," Epstein added.

The outbreak started in Guinea's remote southeast but has spread across the region's porous borders despite aid workers scrambling to help some of the world's weakest health systems tackle the deadly, infectious disease.

In Sierra Leone and Guinea, experts think scores of patients are being hidden as relatives and friends believe hospitalisation is a "death sentence".

In Liberia, health workers have been chased away by armed gangs.