More than 6,300 people have now died from Ebola in the three hardest hit nations in west Africa, the World Health Organization said Monday, as Sierra Leone overtook Liberia as the country with the highest number of cases.

The WHO reported a total of 6,331 deaths in Guinea, Liberia and Sierra Leone since the beginning of the outbreak a year ago, up from 6,055 a few days earlier.

The toll elsewhere remains the same—six in Mali, one in the United States, and eight in Nigeria, which was declared Ebola-free in October.

Across the eight countries affected—including Spain and Senegal, which have both now been declared Ebola free—there have been 6,346 deaths and 17,834 cases.

In Sierra Leone, where an increase in cases in the west of the country has been causing concern, 1,742 deaths out of 7,798 cases were reported as of December 6.

In last week's WHO update, Sierra Leone reported 1,583 deaths out of 7,312 suspected cases.

It is now the country worst affected by the killer virus, overtaking Liberia where 3,177 people had died out of a total of 7,719 cases as of December 3.

In Guinea, where the outbreak started in December last year, 1,412 fatalities out of 2,283 were reported as of December 6.