One of Liberia's most high-profile doctors has died of Ebola, a government official said Sunday, and two American aid workers at a hospital in the West African country have been infected, highlighting the risks facing health workers trying to combat the spread of the deadly virus.

Samuel Brisbane is the first Liberian doctor to die in an outbreak the World Health Organization says has killed 129 people in Liberia, and more than 670 in several West African countries. The WHO confirmed that the Liberian ministry of health had informed the organization that Dr. Brisbane had died. A Ugandan doctor working in the country died this month.

The WHO says the outbreak, the largest yet recorded, has also killed 319 people in Guinea and 224 in Sierra Leone. As of July 23, the total number of cases in the three countries was 1,201, it said.

In Nigeria, officials said on Friday that a Liberian official had died of Ebola after flying from Monrovia to Lagos, Nigeria, via Lome, Togo, the Associated Press reported. The case underscored the difficulty of preventing Ebola victims from traveling given weak screening systems and the fact that the initial symptoms of the disease--including fever and sore throat--resemble those of many other illnesses.

Health workers are at serious risk of contracting the disease, which spreads through contact with bodily fluids. The WHO says the disease isn't contagious until a person begins to show symptoms.

Kent Brantly, an American doctor helping respond to the outbreak in Liberia, is receiving intensive medical treatment there after he was infected with the Ebola virus, a spokeswoman for aid organization Samaritan's Purse said, according to the AP. Dr. Brantly was in stable condition, talking with his doctors and working on his computer while receiving care, she said. Dr. Brantly is medical director for the Samaritan's Purse Ebola Consolidated Case Management Center in Monrovia.

American aid worker Nancy Writebol tested positive at the same medical compound, said Ken Isaacs, of Samaritan's Purse, according to the AP. Ms. Writebol also was in stable condition and receiving intensive treatment, he said.

"It's been a shock to everyone on our team to have two of our players get pounded with the disease," Mr. Isaacs said, the AP reported. Ms. Writebol had been working as a hygienist who decontaminated those entering and leaving the hospital's Ebola care area, he said.

This year, Dr. Brantly was quoted on the dangers facing health workers trying to contain the disease. "In past Ebola outbreaks, many of the casualties have been health-care workers who contracted the disease through their work caring for infected individuals," he said.

Sierra Leone's top Ebola doctor also fell ill with the disease last week, though the country's chief medical officer, Brima Kargbo, said on Sunday that he was "fairly stable and responding well to treatment."

Dr. Brisbane, who once served as a medical adviser to former Liberian President Charles Taylor, was working as a consultant with the internal medicine unit at the country's largest hospital, the John F. Kennedy Memorial Medical Center in Monrovia.

After falling ill with Ebola, he was taken to a treatment center on the outskirts of the capital, where he died, said Tolbert Nyenswah, an assistant health minister quoted by the AP. Under the supervision of health workers, family members escorted the doctor's body to a burial location west of the city, Mr. Nyenswah said.

Another doctor who had been working in Liberia's central Bong County was also being treated for Ebola at the same center where Dr. Brisbane died, Mr. Nyenswah said, adding that the situation "is getting more and more scary."

News of Dr. Brisbane's death first began circulating on Saturday, a national holiday marking Liberia's independence in 1847.

Liberian President Ellen Johnson Sirleaf used her Independence Day address to discuss a new task force to combat Ebola. Information Minister Lewis Brown said the task force would go "from community to community, from village to village, from town to town" to increase awareness.

That a sick Liberian was able to 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 said it was on high alert.

Some security analysts said they were skeptical about the usefulness of these measures, however.

"In Nigeria's case, the security setup is currently bad, so I doubt it will help or have the minimum effectiveness they are hoping for," said Yan St. Pierre, chief executive of Berlin-based security consulting firm Mosecon.

An outbreak in Lagos, a megacity where many lived in cramped conditions, could be a public-health disaster.

There is no known cure for Ebola. 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. But porous borders, satellite outbreaks and widespread distrust of health workers have made the outbreak difficult to bring under control.

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 named Saudata Koroma fell ill. She was forcibly removed from a government hospital by her family, sparking a frantic search that ended Friday. Dr. Kargbo, the country's chief medical officer, said Sunday that