Medecins Sans Frontieres, also known as Doctors Without Borders, said the WHO pledge "needs to translate to immediate and effective action." While the group has deployed some 550 health workers, it said it did not have the resources to expand further.
Doctors Without Borders said its teams are overwhelmed with new Ebola patients in Sierra Leone and that the situation in Liberia is now "dire."
"Over the last weeks, there has been a significant surge in the epidemic - the number of cases has increased dramatically in Sierra Leone and Liberia, and the disease has spread to many more villages and towns," the organization said in a statement. "After a lull in new cases in Guinea, there has been a resurgence in infections and deaths in the past week."
At least 729 people have died since cases first emerged in March: 339 in Guinea, 233 in Sierra Leone, 156 in Liberia and one in Nigeria.
Two American health workers in Liberia have been infected, and an American man of Liberian descent died in Nigeria from the disease, health authorities there say.
Plans were underway to bring the two American aid workers Nancy Writebol and Dr. Kent Brantly back to the U.S. A small private jet based in Atlanta has been dispatched to Liberia. Officials said the jet was outfitted with a special, portable tent designed for transporting patients with highly infectious diseases.
While health officials say the virus is transmitted only through direct contact with bodily fluids, many sick patients have refused to go to isolation centers and have infected family members and other caregivers.
The fatality rate has been about 60 percent, and the scenes of patients bleeding from the eyes, mouth and ears has led many relatives to keep their sick family members at home instead. Sierra Leone is now sending teams door-to-door in search of Ebola patients and others who have been exposed to the disease.
Chan emphasized Friday that the general public "is not at high risk of infection," but also said the Ebola virus should not be allowed to circulate widely.
"Constant mutation and adaptation are the survival mechanisms of viruses and other microbes," she said. "We must not give this virus opportunities to deliver more surprises."
Randy Schoepp, chief of diagnostics at the U.S. Army Medical Research Institute of Infectious Diseases, which is running the only lab in Liberia testing Ebola samples, said: "The virus is getting to large, dense, city areas. We're now getting samples (to test) from all over."
But he said he thinks "we're only seeing a small portion of the cases out there," partly because many drivers are scared to transport vials of blood that may contain Ebola to the lab.
Other countries are taking precautions to prevent the spread of Ebola.
The African Union mission in Somalia canceled a planned troop rotation by Sierra Leonean forces in an effort to prevent Ebola from crossing into the Horn of Africa country, the military said.
Seychelles forfeited an African Cup qualifying game and withdrew from the competition Thursday rather than allow Sierra Leone's soccer team to travel to the Indian Ocean island. And a cyclist from Sierra Leone competed in the Commonwealth Games after testing negative for Ebola.
Nigeria's minister of health, Onyebuchi Chukwu, said Thursday the government has located 10 more people who had primary contact with the man who flew to Lagos, and died there because of Ebola. The government is tracking down the remaining people who had contact with him, he said. As of Friday, 69 people are under surveillance and two are quarantined, Chukwu said.
Meanwhile, families in the United States expect to be reunited as early as this weekend with some of the more than 300 Peace Corps volunteers evacuated from West Africa as a precaution.
"We did really have faith in the Peace Corps that if things would become dangerous they would do what they're now doing," said Mirna Jope of Carmichael, California, whose 25-year-old son called home Thursday after learning he would be leaving Sierra Leone.
A Peace Corps spokeswoman said the organization is working to bring the volunteers home as quickly as possible and following Centers for Disease Control and Prevention guidelines that say people without a known exposure don't need to be screened but should monitor their health. Two workers who have been exposed to the virus still were being monitored.
"The two Peace Corps volunteers who have had contact with an individual who later died of the virus are not symptomatic and are currently isolated and under observation," said spokeswoman Shira Kramer. "When they receive medical clearance for return to the U.S., we will work with them to travel safely back."