GENEVA — The number of people killed by the Ebola outbreak in West Africa has reached at least 2,917, driven by the continuing rapid spread of the disease in Liberia and Sierra Leone, though in Guinea the number of new cases appears to have stabilized, the World Health Organization reported Thursday.

At least 2,909 people have died in Guinea, Liberia and Sierra Leone, with 6,242 reported Ebola cases over all, according to the latest report of the United Nations health body. Nigeria and Senegal have recorded a total of eight deaths and 21 cases of infection.

More than half the total deaths and infections have occurred in Liberia, which reported more than 1,700 new cases in the last 21 days, the W.H.O. said. In Sierra Leone, two districts at the heart of the country’s Ebola outbreak have recorded declining numbers of people infected, but the organization said further investigation was needed to establish whether this was a trend, and in the meantime it reported there had been a sharp rise in new cases in the capital, Freetown.