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.