(Reuters) – The death toll in the world’s worst Ebola epidemic has risen to 5,689 out of 15,935 cases reported in eight countries by Nov. 23, the World Health Organization said on Wednesday.

Almost all cases and all but 15 deaths have been in Guinea, Sierra Leone and Liberia – the three hardest-hit countries, which reported 600 new cases in the past week, the WHO said in its latest update.

“The total number of cases reported in Sierra Leone since the outbreak began will soon eclipse the number reported from Liberia,” it said. The former British colony has reported 6,599 cases against 7,168 in Liberia.

Transmission of the virus remains intense in Sierra Leone, especially in the west and north, with the capital Freetown still the worst affected area, it said.

Sierra Leone appealed to the United States on Wednesday to send military aid to help it battle Ebola as it falls behind its West African neighbors Guinea and Liberia in the fight against the virus.

“Liberia and Sierra Leone report that fewer than 70 percent of patients are isolated, though there is wide variation among districts,” the WHO said, referring to an international target set for Dec. 1. However, some data is out of date, it said.

Isolation is required to halt further spread of the viral hemorrhagic disease, and the aim is to isolate 100 percent of patients by Jan. 1, it added.

Contacts of people known to be infected should be monitored for symptoms including fever, but relatively low numbers being reported “suggest that in districts with high case incidence fewer contacts are currently registered in connection with each new case than is necessary to accurately monitor chains of transmission”, the WHO warned.

Mali has reported 8 Ebola cases, six of them fatal, and 285 contacts exposed to the virus there are being checked, it said.

WHO teams are evaluating the preparedness of neighboring countries to combat Ebola, and visits are planned to the Central African Republic, Niger, and Ethiopia next week, it said.

Peter Piot, a leading specialist on the disease, said on Wednesday that West Africa’s Ebola epidemic could worsen further before abating, but that but new infections should start to decline in all affected countries by the end of the year.

The first Cuban doctor infected with Ebola, evacuated from Sierra Leone to Geneva last week, is improving and responding to treatment, the University Hospital of Geneva said in a statement late on Tuesday. His medical team is “reasonably optimistic”.