An Ebola outbreak that has killed more than 670 people in Africa is now taking a toll on doctors and health care workers battling the deadly disease.

A government official in Liberia said Sunday that one of that country's highest-profile doctors has died in what the World Health Organization (WHO) calls the largest recorded outbreak of the disease.

An American doctor who has been working in Liberia since October 2013 for the North Carolina-based aid organization Samaritan's Purse is receiving intensive medical treatment after he was infected with Ebola, according to a spokeswoman for the organization.

Melissa Strickland said Kent Brantly, 33, was in stable condition, talking with his doctors and working on his computer while being treated. She cautioned that he is "not out of the woods yet." Strickland said patients have a better chance of survival if they receive treatment immediately after being infected, as Brantly did.

Brantly, who is married with two children and is medical director for the Samaritan's Purse Ebola Consolidated Case Management Center in Monrovia, is being treated at a Samaritan's Purse isolation center, according to Strickland.

Samaritan's Purse said later Sunday that a second U.S. citizen, Nancy Writebol, also has tested positive for Ebola. Writebol is employed by mission group SIM in Liberia and was helping a joint SIM/Samaritan's Purse team treating Ebola patients in Monrovia. Writebol is married with two children, the organization said.

The Ebola outbreak in the West African nations of Liberia, Sierra Leone and Guinea has caused more than 670 deaths and more than 1,000 infections, according to the WHO. Ebola is a severe, often fatal illness with a fatality rate of up to 90% and is one of the world's most virulent diseases, according to the WHO. It is transmitted by direct contact with the blood, body fluids and tissues of infected animals or people.

The first Liberian doctor to die of the disease was identified as Samuel Brisbane. He 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.

Brisbane, who once was a medical adviser to former Liberian President Charles Taylor, was taken to a treatment center on the outskirts of the capital after falling ill with Ebola and died there, said Tolbert Nyenswah, an assistant health minister.

He said another doctor who had been working in Liberia's central Bong County also was being treated for Ebola at the same center where Brisbane died.

The situation "is getting more and more scary," Nyenswah said.

A Ugandan doctor working in Liberia, where an Ebola outbreak has killed 129 people, died earlier this month. The current outbreak has claimed the lives of 319 in Guinea and 224 in Sierra Leone.

Last week, the medical humanitarian organization Doctors Without Borders announced that the chief doctor leading the fight against the Ebola epidemic in Sierra Leone, Sheik Umar Khan, had contracted the disease. Three nurses who worked in the same Ebola treatment Center as Khan, 39, are believed to have died from the disease.

Doctors Without Borders says it implements "strict infection control measures" to protect its staff in West Africa against the disease. "As well as the personal protective equipment that our staff wears, we have a series of strict procedures and protocols," says the group's Emergency Coordinator, Marie-Christine Ferir. "Our treatment centers are designed to ensure the safest possible working environment for our staff. There is sufficient space in between patients, clear separation between high-risk and low-risk areas, sufficient lighting, secure waste management and disinfection of the wards."

"Our international staff are rotated every 4-6 weeks to ensure they are not too tired, which helps reduce risk," Ferir says. "Our staff always work in pairs in a buddy system. They look out for each other to make sure the other is not making any mistakes or is too tired. We try wherever possible to administer oral therapy rather than injections, which reduces the risk of needle stick injuries, and also limit the number of blood tests."

Over the weekend, health officials in Nigeria raced to stop the spread of Ebola after a man sick with the disease arrived on a flight in Lagos, Africa's largest city with 21 million people. The man's ability to board an international flight raised new fears that other passengers could carry the disease beyond Africa because of weak passenger inspection and the fact that Ebola's initial symptoms can resemble those of other illnesses.

Contributing: Associated Press