The World Health Organization says Ebola could infect more than 20,000 people across West Africa by the time the outbreak is over.
The U.N. agency gave its assessment Thursday as it launched a $490 million campaign to fight the epidemic that it says has killed at least 1,550 people.
It says the total number of reported cases has topped 3,000, but added the actual number could be two to four times higher.
The majority of the infections have been in Liberia, Sierra Leone and Guinea. The WHO said the outbreak "continues to accelerate," but that most cases are concentrated in a few local areas.
Earlier Thursday, Nigeria's health ministry reported two more cases of Ebola, bringing the country's total to 15.
Officials said Thursday that a man working for the Economic Community of West African States recovered from an Ebola infection without treatment, but that his doctor died last week from the virus.
Six people have now died from Ebola in Nigeria during the latest outbreak.
The WHO said Wednesday that 80 people are also being monitored for Ebola in the Democratic Republic of Congo in an outbreak that is not related to the one in West Africa.
Meanwhile, the U.S. Agency for International Development is providing an additional $5 million to fight the Ebola outbreak in West Africa.
The new funding will be used for health equipment, training of health care workers and support of public outreach campaigns.
USAID has now committed nearly $20 million to combat Ebola since the outbreak was first reported in March.
A shortage of protective equipment is one of the factors contributing to the epidemic.