"She died on arrival and a swab was taken, analyzed in the lab and was confirmed," Nyenswah said. "We are investigating the source."

Liberians hoped they were finally clear of Ebola. But this happened "given the tenacity of the Ebola virus," he said.

The West African country was declared free from transmission Jan. 14. Liberia was first declared free of the disease in May, but new cases emerged two times — forcing officials there to restart the clock each time.

WHO on Twitter Friday reiterated that Ebola is no longer an international health emergency, but said flare-ups, at decreasing frequency, are expected.

Flare-ups have also occurred in Sierra Leone and in Guinea, though health authorities say those are not linked to the original chain of transmission. Guinea's flare-up came months after the outbreak was declared over there and hours after Sierra Leone announced the end of its flare-up.

WHO said there have been eight cases of Ebola and seven deaths in Guinea since late February. The most recent case, an 11-year old girl, is being treated in Nzerekore, and is in stable condition, it said. Six of the deceased are from three generations of the same extended family in Koropara village, it said.

Ebola is spread through direct contact with the bodily fluids of victims or corpses. Traditional funerals, in which mourners touch the body, were a major source of virus transmission during the epidemic in West Africa.

Ebola's symptoms can initially mimic those of other tropical diseases — high fever and fatigue. In its advanced stages, Ebola can cause severe hemorrhaging.

The world's deadliest Ebola outbreak has killed more than 11,300 people, mostly in Sierra Leone, Liberia and Guinea, since December 2013.