The World Health Organization says the past week has seen the highest increase of Ebola cases since the outbreak began, more evidence that the crisis is worsening.

In an update Friday, the U.N. health agency said more than 500 cases were recorded over the past week, by far the worst toll of any week so far.

The vast majority of the cases were in Liberia, but the agency said it was also the highest number of cases in one week for Guinea and Sierra Leone.

WHO warned Thursday that the outbreak in West Africa is accelerating and could eventually infect 20,000 people.

So far, it has killed more than 1,500 of the 3,000 people it has sickened.

Nigeria has also recorded a small number of cases.(AP)