NEW YORK — U.S. health officials on Thursday warned Americans not to travel to the three West African countries hit by an outbreak of Ebola.

The travel advisory applies to nonessential travel to Guinea, Liberia and Sierra Leone, where the deadly disease has killed more than 700 people this year.

“The bottom line is Ebola is worsening in West Africa,” said Dr. Tom Frieden, director of the Centers for Disease Control and Prevention, who announced the travel warning.

He called Ebola “a tragic, dreadful and merciless virus.”

The purpose of the travel warning is to not only protect U.S. travellers, but limit their use of overburdened clinics and hospitals for injuries or other illnesses, he said.

For more than a month, CDC has advised travellers to simply take precautions when in the outbreak region. Thursday’s alert is the highest-level. The World Health Organization, however, has not issued a similar travel warning for the West Africa region. The last time the CDC issued a high-level warning was in 2003 because of a SARS outbreak in Asia.

The current outbreak is the largest since the disease first emerged in Africa nearly 40 years ago. The virus is contagious and is spread by direct contact with blood or bodily fluids from a sick person. Ebola can’t be spread like flu through casual contact or breathing in the same air.

Experts estimate that in this outbreak, about 60 per cent of the people who have gotten sick with Ebola have died — a frightening fatality rate that is among the highest of any disease. There is no vaccine and no specific treatment for it.

Two American aid workers in Liberia have been diagnosed with Ebola, and one of them was getting an experimental treatment.

At the White House, press secretary Josh Earnest said the U.S. is looking into options to bring them back to the U.S. While the U.S. government would facilitate the trip, private companies would be used to transport them.

Late Thursday afternoon, 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.

The hospital, which is near the CDC’s main campus, has a special isolation unit built in collaboration with the CDC. It is one of only four facilities of its kind in the United States.

The CDC has about two dozen staffers in West Africa to help try to control the outbreak. Frieden on Thursday said the CDC will send 50 more in the next month. CDC workers in Africa also are helping at airports to help screen passengers, he said.

The CDC has said that the risk of the Ebola virus coming to the United States remains small. On Monday, the agency sent a health alert to U.S. doctors, updating them about the outbreak. The alert stressed they should ask about foreign travel in patients who come down with Ebola-like symptoms, including fever, headache, vomiting and diarrhea.

Even if someone infected with Ebola came to the U.S., the risk of an outbreak is considered very low, Frieden said. Patients are contagious only when they show symptoms and U.S. hospitals are well equipped to isolate cases and control spread of the virus.

Frieden also noted that relatively few people travel from West Africa to the United States. He said about 10,000 travellers from those countries come to the United States in an average three- or four-month period, and most do not arrive on direct flights.

The CDC has staff at 20 U.S. airports and border crossings. They evaluate any travellers with signs ofdangerous infectious diseases, and isolate them when necessary. The agency is prepared to increase that staffing if needed, he said.

Frieden said a widespread Ebola outbreak in the United States “is not in the cards.”