(Reuters) — The United States said on Wednesday it will begin extra screening of passengers arriving at five of the country's largest airports from West Africa as it increases efforts to prevent the spread of a deadly Ebola outbreak.

The enhanced screening will start at New York's John F. Kennedy International Airport this weekend and be extended next week to Newark Liberty in New Jersey, Washington Dulles, Chicago O'Hare and Hartsfield-Jackson Atlanta.

Combined, those airports receive more than 94% of travelers from Guinea, Liberia and Sierra Leone, the countries hardest hit by Ebola, with JFK accounting for nearly half of them, the U.S. Centers for Disease Control and Prevention said.

The U.S. government has been under pressure from lawmakers to enhance screening and even ban flights from some West African countries after a Liberian national was diagnosed in the United States with Ebola. Thomas Eric Duncan, who traveled to Dallas from Liberia last week, died on Wednesday. He was the first person diagnosed with Ebola on U.S. soil.

Passengers arriving at any of the five U.S. airports from Sierra Leone, Guinea and Liberia will get extra screening whether they arrived on a direct flight or connected in another country to a U.S.-bound flight, said a senior U.S. official who asked not to be named.

Customs and Border Patrol officers will escort travelers from the three countries to a screening area, where trained staff will observe them for signs of illness, ask about their health and possible exposure to Ebola and take their temperature with a non-invasive device, the CDC said.

If they have fever, any other symptoms or give answers on a health questionnaire that point to possible Ebola exposure, they will be evaluated by a CDC quarantine station public health officer. Travelers who are deemed in need of further evaluation will be referred to local public health authorities.

“We believe these new measures will further protect the health of Americans, understanding that nothing we can do will get us to absolute zero risk until we end the Ebola epidemic in West Africa,” CDC director Dr. Thomas Frieden said in a statement.

U.S. officials stressed that the new program supplements exit screening measures in place in the affected countries. In two months, 36,000 people have been screened, of whom 77 were not permitted to board a flight. The CDC said none of the 77 were infected with Ebola.

“This is another layer of screening that individuals who are coming from those three countries will have to go through,” White House spokesman Josh Earnest told reporters. “And what that will do is it will add some confidence in our ability to continue to protect the American people from the Ebola outbreak.”

According to the World Health Organization, the worst Ebola outbreak on record has killed at least 3,879 people from among 8,033 confirmed, probable and suspected cases since it was identified in Guinea in March.

Shares in major U.S. air carriers declined on Wednesday. Delta Air Lines dropped 1.5%, United Airlines fell 3.1% and American Airlines shed 2.8%.