GENEVA (REUTERS) - Ebola has spread further in Guinea and Sierra Leone, with 36 cases in the past week, four times as many cases as the week before, the World Health Organization (WHO) said on Tuesday.

The findings were a reminder that "this virus will not go quietly and it will take an extraordinary effort to finish the job," the WHO's special representative for Ebola, Bruce Aylward, told a technical briefing attended by health ministers.

"With the start of the rainy season today, the doubling of effort will be that much more difficult, that much more important," he added.

Liberia was declared Ebola-free earlier this month.