The death toll from the worst recorded Ebola outbreak in history surpassed 700 in West Africa as security forces went house-to-house in Sierra Leone’s capital Thursday looking for patients and others exposed to the disease.

Fears grew as the United States warned against travel to the three infected countries — Guinea, Sierra Leone and Liberia — and Sierra Leone’s soccer team was blocked from boarding a plane in Nairobi, Kenya, that was to take them to the Seychelles for a game on Saturday. Airport authorities in Kenya said Seychelles immigration told them to prevent the team from traveling.

Almost half of the 57 new deaths reported by the World Health Organization occurred in Liberia, where two Americans, Dr. Kent Brantly of Texas and Nancy Writebol, a North Carolina-based missionary, are also sick with Ebola.

At the White House, press secretary Josh Earnest said the U.S. is looking into options to bring them back to the U.S. Officials at Atlanta’s Emory University Hospital said they expected one of the Americans to be transferred there “within the next several days.” The hospital declined to identify which aid worker, citing privacy laws.

Writebol is in stable but serious condition and is receiving an experimental treatment that doctors hope will better address her condition, according to a statement released by SIM, a Christian missions organization. Her husband, David, is close by but can only visit his wife through a window or dressed in a haz-mat suit, the statement said.

“There was only enough (of the experimental serum) for one person. Dr. Brantly asked that it be given to Nancy Writebol,” said Franklin Graham, president of Samaritan’s Purse, another aid organization that has been working in Liberia during the Ebola crisis.

Brantly, who works for the aid group, did receive a unit of blood from a 14-year-old boy who had survived Ebola because of the doctor’s care, Graham said in a statement.

“The young boy and his family wanted to be able to help the doctor who saved his life,” he said.

Giving a survivor’s blood to a patient might be aimed at seeing whether any antibodies the survivor made to the virus could help someone else fight off the infection. This approach has been tried in previous Ebola outbreaks with mixed results.

No further details were provided on the experimental treatment. There is currently no licensed drug or vaccine for Ebola, and patients can only be given supportive care to keep them hydrated. There are a handful of experimental drug and vaccine candidates for Ebola and while some have had promising results in animals including monkeys, none has been rigorously tested in humans.

The disease has continued to spread through bodily fluids as sick people remain out in the community and cared for by relatives without protective gear. People have become ill from touching sick family members and in some cases from soiled linens.

In Sierra Leone, which borders Liberia to the northwest, authorities are vowing to quarantine all those at home who have refused to go to isolation centers. Many families have kept relatives at home to pray for their survival instead of bringing them to clinics that have had a 60 percent fatality rate. Those in the throngs of death can bleed from their eyes, mouth and ears.

Rosa Crestani, Ebola emergency coordinator for Doctors Without Borders, also known as Medecins Sans Frontieres, said it is “crucial” at this point to gain the trust of communities that have been afraid to let health workers in and to deploy more medical staff.

“The declaration of a state of emergency in Sierra Leone shows a recognition of the gravity of the situation, but we do not yet know what this will mean on the ground. What we can say is that it will be difficult to implement due to the fact that the cases are dispersed over such a large area, and that we currently do not have a clear picture of where all the hotspots are,” she said.

Liberia’s president on Wednesday also instituted new measures aimed at halting the spread of Ebola, including shutting down schools and ordering most public servants to stay home from work.

“It could be helpful for the government to have powers to isolate and quarantine people and it’s certainly better than what’s been done so far,” said Dr. Heinz Feldmann, chief of virology at U.S. National Institute of Allergy and Infectious Diseases. “Whether it works, we will have to wait and see.”

Dr. Unni Krishnan, head of disaster preparedness and response for the aid group Plan International, said closing schools could help as they bring large numbers of children together, which can amplify infection rates.

“Door-to-door searches are not going to be easy,” he said. “What will help is encouraging people to come forward when they see symptoms and seek medical help.”

The U.S. Peace Corps also was evacuating hundreds of its volunteers in the affected countries. Two Peace Corps workers are under isolation outside the U.S. after having contact with a person who later died from the Ebola virus, a State Department official said.

In Moberly, Missouri, Liz Sosniecki said she got a call from her 25-year-old son, Dane, a Peace Corps volunteer in Liberia. He had not been exposed to Ebola and expressed disappointment about leaving just six weeks after he arrived.

“He said, ‘I’m coming home.’ Sorry,” she said, beginning to cry. “I’m a little emotional. It’s a relief.”

The last time the U.S. Centers for Disease Control and Prevention issued such a travel warning during a disease outbreak was in 2003 because of SARS in Asia.

Ebola now has been blamed for 729 deaths in four West African countries this year: 339 in Guinea, 233 in Sierra Leone, 156 in Liberia and one in Nigeria.

The World Health Organization is launching a $100 million response plan calling for the deployment of several hundred additional health workers to help the strained resources in deeply impoverished West Africa, where hospital and clinics are ill-equipped to cope with routine health threats let alone the outbreak of a virulent disease like Ebola.

Among the deaths announced this week was that of the chief doctor treating Ebola in Sierra Leone, who was buried Thursday. The government said Dr. Sheik Humarr Khan’s death was “an irreparable loss of this son of the soil.” The 39-year-old was a leading doctor on hemorrhagic fevers in a nation with very few medical resources.

The Ebola cases first emerged in Guinea back in March, and later spread across the borders to Liberia and Sierra Leone. Outbreaks of the virus in previous years had occurred in other parts of Africa.

The current outbreak is now the largest recorded in world history, and has infected three African capitals with international airports. Officials are trying to step up screening of passengers, though an American man was able to fly from Liberia to Nigeria, where authorities say he died days later from Ebola.

Experts say the risk of travelers contracting it is considered low because it requires direct contact with bodily fluids or secretions such as urine, blood, sweat or saliva. Ebola can’t be spread like flu through casual contact or breathing in the same air.

Patients are contagious only once the disease has progressed to the point they show symptoms, according to the World Health Organization. The most vulnerable are health care workers and relatives who come in much closer contact with the sick.

In Liberia, authorities say 28 out of the 45 health workers who have contracted the disease so far have died.

(AP)