FREETOWN, Sierra Leone — 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 hazmat 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, none has been rigorously tested in humans.