Health officials in Sierra Leone fear a major Ebola outbreak may have gone largely unreported until now in a remote district where the World Health Organization (WHO) said scores of bodies piled up in a hospital.

The WHO said on Wednesday that it had sent a response team to the diamond-rich Kono district following a worrying spike in reported Ebola cases in the district, which lies along the country's eastern border with Guinea.

"They uncovered a grim scene," the U.N. health agency said in a statement. "In 11 days, two teams buried 87 bodies, including a nurse, an ambulance driver, and a janitor drafted into removing bodies as they piled up."

Twenty-five people had died in a hastily cordoned off section of the local hospital in the five days before the team arrived. They found that villages scattered across eight of the area's 15 chiefdoms had been hit by Ebola.

Officially the district of over 350,000 inhabitants had reported 119 cases up to Dec. 9.

"We are only seeing the ears of the hippo," said Dr. Amara Jambai, Sierra Leone's Director of Disease Prevention and Control, expressing concern that the official figures underrepresented the size of the outbreak in Kono.

The worst Ebola epidemic on record has killed 6,388 people out of 17,942 cases in Guinea, Liberia and Sierra Leone according to WHO data.

The toll continues to grow, fuelled principally by new infections in Sierra Leone, but health officials worry that the true scale of the epidemic may be even larger.

The team's findings in Kono did not appear to be reflected in the WHO's most recent data on the epidemic published on Wednesday, which showed 24 cases reported in the district in the week leading up to Dec. 7.

Sierra Leone recently overtook neighbouring Liberia for the highest number of Ebola cases, recording 7,897 since the epidemic was first identified earlier this year.

But it has registered just 1,768 deaths, well below Liberia's 3,177 dead, raising concerns that some fatalities may not be reflected in the figures.

"It is difficult to put an exact figure on the deaths," Sierra Leone's health minister Abu Bakarr Fofanah told Reuters in an interview in Geneva, explaining that his country was only counting deaths from laboratory confirmed Ebola cases.

"They are adding suspected cases, so that is causing the discrepancies in the results. We are going by the textbook," he said.