The World Health Organization (WHO) on Thursday warned against complacency about the deadly Ebola virus, saying that the crisis isn't over.

"We have to keep our guard up," Sylvie Briand, head of epidemiology at the UN's health agency, told a stock-taking meeting on Ebola in Paris.

Briand said the situation in Guinea, one of the three West African states where Ebola has killed more than 11,000 people, left her "deeply concerned."

Briand warned against hopes that a line could be drawn under the epidemic. stressing that "the crisis isn't over".

"I am afraid there could be another wave that could spread to neighbouring countries," she said.

The WHO official noted that Ebola emerged in Guinea at the end of 2013 and by April of 2014 had practically disappeared there.

It then dramatically bounced back a couple of months later before spreading to neighbouring Liberia and Sierra Leone, with a scattering of cases in Mali, Nigeria and Senegal.

Liberia was recently declared Ebola-free but Sierra Leone and Guinea remain vulnerable to a resurgence of the lethal virus, the WHO said last week.

Briand was speaking at a two-day conference at the Pasteur Institute in Paris that will assess progress towards treatments and vaccines for Ebola.