More than 6,800 people have now died from the Ebola virus, almost all of them in west Africa, the World Health Organization said.

The UN health agency reported that as of December 13, there had been 18,464 cases of infection from the deadly virus in Guinea, Liberia and Sierra Leone, and 6,841 people had died, an increase of 258 deaths in three days.

The last update WHO provided showed that as of December 10, there had been 18,188 cases of infection from the deadly virus in Guinea, Liberia and Sierra Leone, of whom 6,583 people had died.

WHO did not provide an update of cases in other countries, but last week said the death toll remained the same: six in Mali, one in the United States, and eight in Nigeria, which was declared Ebola free in October.

Spain and Senegal, which have both been declared free from Ebola, meanwhile counted one case each, but no deaths.

Sierra Leone, which last week overtook Liberia as the nation with the most infections, counted 8,273 cases and 2,033 deaths on December 13.

The toll up until December 10 stood at 8,069 cases and 1,899 deaths.

Liberia, long the hardest-hit country, has meanwhile seen a decrease in the rate of transmission in recent weeks.

As of December 9, the country counted 7,797 cases and 3,290 deaths, up from the 7,765 infections and 3,222 deaths tallied two days earlier.

In Guinea, where the outbreak started nearly a year ago, 2,394 Ebola cases and 1,518 deaths were recorded as of December 13.

Three days earlier, the country counted 2,354 Ebola cases and 1,462 deaths.

Ebola, one of the deadliest viruses known to man, is spread only through direct contact with the bodily fluids of an infected person showing symptoms such as fever or vomiting.

People caring for the sick or handling the bodies of people infected Ebola are especially exposed.

As of the latest figures up to December 7, a total of 639 healthcare workers were known to have contracted the virus, and 349 of them had died, WHO said.