Authorities in Sierra Leone have imposed a two-week lockdown in the eastern district of Kono after health workers uncovered a surge of Ebola infections in the area where the epidemic was thought to be largely under control.

The worst outbreak of Ebola on record has killed 6,533 people in the three West African countries most hit by the disease -- Sierra Leone, Liberia and Guinea -- and infected 18,118 people, the World Health Organization said on Thursday

Sierra Leone, with a shortage of treatment centers and trained staff, has overtaken Liberia as the worst affected nation, and until now, the recent spread was believed to be centered on western areas around the capital Freetown.

However, the WHO said on Wednesday that it had found bodies piled up at the only hospital in Kono, a district of about 350,000 people bordering Guinea.

Officials from the WHO, health ministry and U.S. Centers for Disease Control and Prevention (CDC) discovered 87 bodies had been buried in 11 days.

Kono District Ebola Response Center said it was placing the area on lockdown, allowing only essential vehicles in and out and introducing a night-time curfew.

Sierra Leone's government said on Wednesday it was working with the United Nations in Kono and the International Federation of the Red Cross was setting up a treatment center there. The remote area has only one ambulance to transport the sick and blood samples for testing.

But in Liberia, medical charity Medecins Sans Frontieres (MSF) said it was withdrawing from northern Lofa County, a former Ebola hotspot, after no new patients were recorded at its treatment center in Foya since Oct. 30, allowing the center's staff to be redeployed.

Ettore Mazzanti, MSF Project Coordinator in Foya, said efforts to contain the outbreak had been helped by explaining to local people how to avoid the virus, which has no known cure and is transmitted through the bodily fluids of sick people.

Scientists are racing to develop Ebola vaccines.

The Ebola response in Sierra Leone has been dogged by strikes by healthcare staff over pay and working conditions.

Despite government claims that it had reached a deal with junior doctors, Dr Jeredine George, president of the Junior Doctors' Association, told Reuters that its members would strike for a fourth day on Thursday.

They are demanding a specialized Ebola treatment clinic for Sierra Leonean doctors, 10 of whom have died since the outbreak began. Deputy Health Minister Madinatu Rahman has said plans are underway to get such a clinic set up this month.