The UN health agency said in a statement released on Thursday that the 57 deaths were recorded between Thursday and Sunday last week in Guinea, Liberia, Nigeria and Sierra Leone.

The statement added that 122 new cases were detected over those four days, pushing the total number of confirmed and likely infected cases from the epidemic so far to 1,323.

The trend in Guinea, Liberia and Sierra Leone "remains precarious with ongoing... transmission of [the] infection,” WHO noted.

Guinea is the worst-hit country as its officials reported 20 more deaths apparently from the disease in the last four days of last week, taking its national fatality figure to 339.

Twenty seven more people lost their lives in Liberia, whose total national death toll stands at 156, while Sierra Leone reported nine more deaths for a total dead of 233.

Nigeria, which is the most populous country in Africa, also reported its first death from Ebola in that period.

Meanwhile, the UN agency has said that it "does not recommend any travel or trade restrictions” be applied to Guinea, Liberia, Nigeria, or Sierra Leone at this time.

There is currently no known cure for Ebola, a form of hemorrhagic fever whose symptoms are diarrhea, vomiting and bleeding.

The virus spreads through direct contact with infected blood, feces or sweat. It can also be spread through sexual contact or the unprotected handling of contaminated corpses.

Ebola remains one of the world’s most virulent diseases, which kills between 25 to 90 percent of those who fall sick.