WATCH: Questions are now being asked about how prepared North American hospitals are to deal with an Ebola case. And as Jackson Proskow reports, many people are demanding travel restrictions to stop the deadly virus from spreading.

WASHINGTON – Health officials said Thursday they still don’t know how two Dallas nurses caught Ebola from a patient, as criticism increased from lawmakers who questioned whether the U.S. is prepared to stop the deadly virus from spreading in the country.

The revelation that one of the hospital nurses was cleared to fly on a commercial airline the day before she was diagnosed raised new alarms about the American response to the Ebola outbreak in West Africa. The death toll is expected to climb above 4,500 in Africa, all but a few within Liberia, Sierra Leone and Guinea, the World Health Organization said.

In Sierra Leone, the government announced the virus had infected two people in the last part of the country that had been free of the disease, in the mountainous north, despite aggressive steps to keep it at bay.

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President Barack Obama authorized a call-up of reserve and National Guard troops in case they are needed. His executive order would allow more forces than the up-to 4,000 already planned to be sent to West Africa, and for longer periods of time.

A ban on travel to the U.S. from the Ebola-stricken countries, sought by some Republican lawmakers, is not under consideration, White House spokesman Josh Earnest said Thursday.

The president met into the evening with top aides and health officials at the White House, declaring afterward that he had no “philosophical objection” to imposing a travel ban on West Africa but had been told by health and security experts that it would be less effective than measures already in place — and perhaps would be counterproductive.

He said a ban could result in people trying to hide where they were coming from and thus becoming less likely to be screened.

He said it may be appropriate to appoint an additional person to lead the anti-Ebola effort in the U.S., a response to calls that he name an Ebola “czar.”

U.N. Secretary-General Ban Ki-moon made another urgent appeal for funds, saying that a trust fund he launched to provide fast and flexible funding for the fight against Ebola has only received $100,000 of the $20 million pledged.

U.N. spokesman Stephane Dujarric said the trust fund is part of a nearly $1 billion U.N. appeal for humanitarian needs in West Africa.

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Nina Pham, a nurse who contracted Ebola after treating a Liberian man in Dallas, was being flown to the National Institutes of Health outside Washington on Thursday, while a second nurse, Amber Joy Vinson, has already been transferred to a biohazard infectious disease centre at Emory University Hospital in Atlanta.

The two nurses had been involved in providing care to Thomas Duncan, who died of Ebola last week.

In Washington, the chairman of a House committee cited “demonstrated failures” in the government’s response. Rep. Tim Murphy said during a hearing that the “trust and credibility of the administration and government are waning as the American public loses confidence each day.”

Dr. Tom Frieden, director of the Centers for Disease Control and Prevention, testified that despite the latest incidents, “we remain confident that our public health and health care systems can prevent an Ebola outbreak here.”

In his prepared testimony, the Texas hospital’s chief clinical officer, Dr. Daniel Varga, admitted the facility had made mistakes in Duncan’s initial treatment, and he apologized for that. Duncan was initially sent home from the emergency room with antibiotics for his high fever, despite saying he’d come from Liberia.

In Europe, Spain’s government is wrestling with similar questions. The condition of a nursing assistant infected with Ebola at a Madrid hospital appeared to be improving, but a person who came in contact with her before she was hospitalized developed a fever and was being tested Thursday.

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That second person is not a health care worker, a Spanish Health Ministry spokesman said.

To this point, only hospital workers — the Madrid nursing assistant and the two nurses in Dallas — had been known to have contracted Ebola outside West Africa during the outbreak that began in March.

France said that on Saturday it will begin screening passengers who arrive at Paris’ Charles de Gaulle airport on the once-daily flight from Guinea’s capital.

In the U.S., Customs and health officials at airports in Chicago, Atlanta, suburban Washington and Newark, New Jersey, were to begin taking the temperatures of passengers from the three hardest-hit West African countries Thursday. The screenings, using no-touch thermometers, started Saturday at New York’s Kennedy International Airport.

With hospitals and airports on heightened alert, Frieden said the CDC is receiving hundreds of requests for help in ruling out Ebola in travellers. So far 12 cases merited testing, he said.

Frieden said investigators are trying to figure out how the nurses caught the virus from Duncan. In the meantime, he said, their cases show a need to strengthen the infection-control procedures that “allowed for exposure to the virus.”

Duncan’s death and the sick health care workers in the U.S. and Spain “intensify our concern about the global health threat,” said Dr. Anthony Fauci, director of the Institute of Allergy and Infectious Diseases at the National Institutes of Health.

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He said two Ebola vaccine candidates were undergoing a first phase of human clinical testing this fall. But he cautioned that scientists were still in the early stages of seeking new treatments or a vaccine.

Dallas County’s top public health epidemiologist, Dr. Wendy Chung, confirmed Thursday that she spent time at Duncan’s beside and that she is among those potentially exposed to the virus. She is undergoing monitoring for any signs of the potentially deadly disease.

A nurse at the Dallas hospital on Thursday described a “chaotic scene” when the hospital faced Duncan, its first Ebola patient.

Briana Aguirre, who has helped treat the first nurse who was infected, told NBC’s “Today” show she felt exposed in the protective gear the hospital provided.

The hospital said it used the protective gear recommended by the CDC and updated the equipment as CDC guidelines changed. Because nurses complained that their necks were exposed, the hospital ordered hoods for them, according to a statement from Texas Health Presbyterian.

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Associated Press writers Emily Schmall and Nomaan Merchant in Dallas and Connie Cass and Calvin Woodward in Washington contributed to the report.