WASHINGTON >> Warning that Americans are losing faith in their government’s ability to stop Ebola, Republican lawmakers on Thursday pressed for a ban on travel to the U.S. from the West African outbreak zone. The White House resisted the idea and tried to tamp down fear as the pool of Americans being monitored expanded.

While a contentious congressional hearing focused on the three cases of Ebola diagnosed within the U.S., the World Health Organization said the outbreak in West Africa was on pace to top 4,500 deaths this week alone. President Barack Obama authorized the call-up of reserve and National Guard troops in case they are needed as part of the up-to-4,000 military personnel being sent to help with the crisis in West Africa.

Called to Capitol Hill for special hearing, federal health officials emphasized the importance of stopping the virus in Liberia, Sierra Leone and Guinea to protect Americans and the rest of the world from its spread.

"You’re right, it needs to be solved in Africa. But until it is, we should not be allowing these folks in, period," replied Rep. Fred Upton, R-Mich. He called for a ban on the 100 to 150 people who fly into the U.S. each week from the three nations at the heart of the outbreak.

"People’s lives are at stake, and the response so far has been unacceptable," declared Upton, chairman of the Energy and Commerce Committee.

At the White House, spokesman Josh Earnest rejected a travel ban. He said the U.S. was already taking the necessary steps to protect the public, because passengers are screened as they depart West Africa and most are checked for fever again when they arrive at a U.S. airport.

Besides, he said, imposing a ban might lead travelers "to go underground and to seek to evade this screening and to not be candid about their travel history in order to enter the country."

Earnest said the chances for a widespread outbreak in the U.S. remain "exceedingly low," despite shortcomings in the government’s initial responses. Health officials said the same.

Frieden told lawmakers that investigators still don’t know how two Dallas nurses caught Ebola while caring for a Liberian man who died at their hospital. Thomas Eric Duncan was the first person diagnosed with Ebola in the United States since the West African outbreak began in March.

To protect other medical workers while the investigation continues, Frieden said, the CDC is focusing on improving safety procedures.

He said one of the sick nurses was being moved Thursday from the Texas hospital to a specialized federal facility in Maryland. The other nurse has been transferred to an Atlanta hospital that has one of only four bio-containment units in the U.S.

Nations and global bodies continued to grapple with the crisis:

— In Sierre Leone, the government said that two cases had turned up in what was the country’s last untouched district. The mountainous Koinadugu district had been the only place in Sierra Leone "where you can go and breathe a sigh of relief, said John Caulker, the executive director of the nonprofit Fambul Tok. "To know that now in the whole country no district is safe is heartrending."

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

— In Geneva, the U.N. high commissioner for human rights, Zeid Raad al-Hussein, paired the Ebola outbreak and the Islamic State group as "twin plagues" that will cost the world many billions of dollars to overcome.

— The United Nations made an urgent appeal for more money to fight the disease. A U.N. trust fund launched to raise $1 billion has taken in only $20 million, and most has been spent. Secretary-General Ban Ki-moon said, "Ebola is a huge and urgent global problem that demands a huge and urgent global response."

— France said 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. on Thursday, Customs and health officials at airports in Chicago, Atlanta, suburban Washington and Newark, New Jersey, were beginning to take the temperatures of passengers from the three West African countries. The screenings, using no-touch thermometers, started Saturday at New York’s Kennedy International Airport.

Obama, who directed his administration to respond more aggressively to the virus at home, made calls about Ebola to foreign heads of state as well as congressional leaders at home. He canceled a campaign trip for the second day in a row to focus on the outbreak.

The revelation that one of the Dallas nurses was cleared to fly on a commercial airline the day before she was diagnosed generated much criticism on Capitol Hill.

Seven people in northeast Ohio were in voluntary quarantine because they had contact with the nurse, Amber Joy Vinson, who visited family in the Akron area last weekend before flying from Cleveland back to Dallas.

Some school and hospital employees also were staying home amid concerns that they might have had contact with her, and a store where she shopped was closed Thursday as a precaution.

Still, health officials in Ohio emphasized that Vinson didn’t show symptoms during her visit and therefore shouldn’t have been contagious yet. The disease isn’t airborne; it’s spread through direct contact with bodily fluids.

As a result of the failure to protect the two nurses from infection and other mistakes, "the American public loses confidence each day," said Rep. Tim Murphy, R-Pa., chairman of the House panel conducting Thursday’s hearing.

Frieden offered assurances that Americans who haven’t traveled recently to West Africa need not worry about catching the deadly disease.

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Associated Press writers Emily Schmall and Nomaan Merchant in Dallas and Erica Werner and Josh Lederman in Washington contributed to the report.