Besides Brantly and the two doctors in Liberia, Sierra Leone's top Ebola doctor and a doctor in Liberia's central Bong County have also fallen ill.
The situation "is getting more and more scary," said Nyenswah, the country's assistant health minister.
Meanwhile, the fact that a sick Liberian could board a flight to Nigeria raised new fears that other passengers could take the disease beyond Africa.
Nigeria's international airports were screening passengers arriving from foreign countries, and health officials were also working with ports and land borders to raise awareness of the disease. Togo's government also said it was on high alert.
Security analysts were skeptical about the usefulness of these measures.
"In Nigeria's case, the security set-up is currently bad, so I doubt it will help or have the minimum effectiveness they are hoping for," said Yan St. Pierre, CEO of the Berlin-based security consulting firm MOSECON.
An outbreak in Lagos, a megacity where many lived in cramped conditions, could be a major public health disaster.
The West Africa outbreak is believed to have begun as far back as January in southeast Guinea, though the first cases weren't confirmed until March.
Since then, officials have tried to contain the disease by isolating victims and educating populations on how to avoid transmission, though porous borders and widespread distrust of health workers have made the outbreak difficult to bring under control.
News of Brisbane's death first began circulating on Saturday, a national holiday marking Liberia's independence in 1847.
President Ellen Johnson Sirleaf used her Independence Day address to discuss a new taskforce to combat Ebola. Information Minister Lewis Brown said the taskforce would go "from community to community, from village to village, from town to town" to try to increase awareness.
In Sierra Leone, which has recorded the highest number of new cases in recent days, the first case originating in Freetown, the capital, came when a hairdresser, Saudata Koroma, fell ill. She was forcibly removed from a government hospital by her family, sparking a frantic search that ended Friday. Kargbo, the chief medical officer, said Sunday that Koroma died while being transported to a treatment center in the east of the country.
Associated Press writers Colleen Slevin in Denver, Clarence Roy-Macaulay in Freetown, Sierra Leone, and Heather Murdock in Lagos, Nigeria, contributed reporting.
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