The number of deaths from Ebola has risen to 9,152, a sharp increase following weeks in which the outbreak appeared to be weakening.
The death toll reported Tuesday by the World Health Organization represents a jump of nearly 150 deaths since the agency's last update three days earlier.
The WHO said the number of new cases climbed by 303, with 136 new cases in Liberia, 113 in Sierra Leone, and 54 in Guinea.
Dr. David Nabarro, the U.N. special envoy on Ebola, told reporters in Geneva that the new numbers showed the outbreak was not yet under control. He said the goal is to reduce the number of new cases to zero.
"Good progress is being made, but the outbreak still represents a grave threat," Nabarro said, "and we really hope that there will be no complacency in anybody involved in the response. We have to really work hard to get zero cases, zero transmissions."
Health experts have cautioned West Africans against becoming complacent about the disease. The WHO recently said a single unsafe burial in Guinea last month caused 11 confirmed Ebola cases.
Those killed by the virus remain contagious and must be buried by workers in protective equipment.
At the United Nations on Tuesday, a letter from three world leaders said the Ebola outbreak had exposed the "weakness'' of international crisis response.
The letter, from German Chancellor Angela Merkel, Norwegian Prime Minister Erna Solberg and Ghanaian President John Mahama, was given to U.N. Secretary-General Ban Ki-moon. It asked the U.N. chief to create a high-level panel and to commission a report on how nations can react more quickly and with better coordination in the face of disaster.
A large-scale trial of two potential Ebola vaccines began earlier this month in Liberia. Organizers of the study, led by the U.S. National Institutes of Health, aim to enlist a total of around 27,000 healthy men and women for the trial.
The WHO has said there is an urgent need to end the outbreak before the wet season begins and access to remote areas becomes more difficult.