World Health Organization (WHO) Director-General Dr. Margaret Chan answers questions at a briefing to discuss the Ebola outbreak in West Africa at the UN Foundation in Washington September 3, 2014. [Photo/Agencies]

Since March, more than 3,500 confirmed or probable cases of the disease have been reported and more than 1,900 people have died, Dr. Margaret Chan, director-general of the WHO, told reporters at a Washington news conference.

Chan said overwhelming fear of Ebola was making it difficult to recruit the foreign medical teams needed to mount an effective response. "That's the reality," she said.

She said the WHO was seeking to gain air and sea access to the affected countries, which have become increasingly isolated as airlines and boats refuse to land or dock for fear of contagion.

Dr. David Nabarro, the senior United Nations Coordinator for Ebola, told the news conference the international effort to contain the outbreak needed to be scaled up three- to four-fold, at a cost of at least $600 million.

That includes increasing the number of motorcycles, ambulances and other vehicles available to transport patients to medical facilities; increasing the supply of protective equipment, gloves and gowns; providing hazard pay and other incentives for local workers; and taking steps to protect local economies from collapse.

Dr. Keiji Fukuda, the WHO assistant director-general for health security, said several thousand medical personnel would be needed to treat the sick as the outbreak grew along with several hundred international experts to help run laboratories and train healthcare workers.

In Liberia on Tuesday, the government began offering a $1,000 bonus to any healthcare worker who agreed to work in Ebola treatment facilities.

Neither the WHO nor the United Nations can force airlines to land in affected countries. Chan said the WHO was in discussions with commercial airline associations and others to address their concerns.

The overall fatality rate of the current outbreak is 51 percent, ranging from a low of 41 percent in Sierra Leone to a high of 66 percent in Guinea, WHO said.

Countries affected by the epidemic include Guinea, Liberia, Nigeria, Senegal and Sierra Leone. An outbreak in the Democratic Republic of Congo is unrelated to and independent of the West African epidemic, Chan said.

The U.S. government "has been a very strong supporter" of WHO's efforts in the outbreak, she added, naming countries including China, South Africa, Switzerland, the United Kingdom, France, Kuwait, and Canada as providing logistical, medical or other support.

Those efforts continue to fall short, however. Most new Ebola infections are occurring in the community as families care for patients who have no place to go and often refuse to be identified to public health workers, Fukuda said.