Health officials are now focused on ending the biggest ever Ebola outbreak rather than just slowing the spread of the deadly virus, the World Health Organisation says.

The UN health agency said the three most affected countries - Guinea, Sierra Leone and Liberia - have reported fewer than 100 cases in the past week, for the first time since June.

However, the WHO said only 30% of new cases in Guinea are known contacts of Ebola patients, meaning officials have no idea how most patients are getting infected.

In Liberia, that figure is 50%, while no data is available for Sierra Leone. The death rate for patients who reach hospital is about 60%.

Researchers at the Institut Pasteur in Paris are studying whether the virus has undergone any significant mutations since the outbreak began. Viruses like Ebola evolve constantly and it is hard to tell if genetic changes make a difference to how people get infected or how sick they become.

The outbreak is believed to have killed more than 8,000 people since it started in March.

Bruce Aylward, who is leading the WHO Ebola response, warned that despite the progress made, the virus is not under control.

"This is like being in bed with two cobras, and one of them is dead," Dr Aylward said. "You still have an incredibly dangerous situation." He added that Ebola outbreaks often come in waves.

"We cannot say the epidemic is under control," said Jerome Oberreit, secretary general of Doctors Without Borders.

At a WHO Ebola meeting on Sunday, Mr Oberreit said there is virtually no sharing of information about the risks of Ebola cases crossing the borders among the three countries in west Africa.

He said surveillance teams lack even basic resources to track down Ebola patients and blamed "international negligence" for the inability to contain the outbreak.