WASHINGTON — Facing renewed criticism about the U.S. response to Ebola, President Barack Obama is conceding it may make sense to have a single person lead the administration’s effort. But he says imposing a travel ban from disease-ravaged West Africa, as Republicans have demanded, would be counterproductive.

In Dallas, the epicenter of Ebola in the U.S., officials took a tougher approach toward monitoring dozens of health care workers who were exposed to the virus while treating an infected patient who later died. The health care workers were asked to sign legally binding documents agreeing not to go to public places or use public transportation.

Those who break the agreement could face undisclosed sanctions.

The move came after two nurses who had treated Thomas Eric Duncan at Texas Health Presbyterian Hospital were diagnosed with Ebola and the disclosure one of them had flown roundtrip between the Dallas area and Cleveland before her diagnosis.

Self-monitoring was extended Thursday to people who took the same outbound flight as nurse Amber Vinson; it had been imposed earlier for passengers on the return trip. Another group being contacted: shoppers at the Akron bridal shop Vinson visited that Saturday.

Both nurses have been transferred from the Texas hospital where they treated Duncan and became infected. Nina Pham was transferred to the National Institutes of Health in Bethesda, Md., and Vinson to Emory University Hospital in Atlanta. The two facilities are among four in the United States that have specialized isolation units.

The chief clinical officer at the hospital, Dr. Daniel Varga, said the hospital was caught short when Duncan came to the institution “with non-specific symptoms.”

“I think we all in the health-care community underestimated the challenge of diagnosis,” he said on ABC’s “Good Morning America” today.

He also said the two nurses who contracted the disease had followed standard hospital procedure. “We have no indication that Nina or Amber had any break in protocol. We were working with the best information we had,” Varga said.

A video released by Texas Health Presbyterian Hospital showed Pham in her hospital isolation ward before her transfer, speaking to her physician, Dr. Gary Weinstein. At one point Pham begins crying, and Weinstein, dressed in full personal protection gear hands her a tissue. “I love you guys,” she says. “We love you, Nina,” responds Weinstein.

The two nurses have been the only cases of transmission in the U.S. Duncan was exposed to the virus in Liberia and was diagnosed after traveling to Texas.

Government officials said early today a Dallas health care worker who handled a lab specimen from Duncan is on a Caribbean cruise ship where she has self-quarantined and is being monitored for any signs of infection. State Department spokesman Jen Psaki said in a statement the woman had shown no signs of the disease.

The government is working to return the woman and her husband to the U.S. before the ship completes its cruise. When the woman left the U.S. on the cruise health officials were requiring only self-monitoring, Psaki said.

The magnitude of the Ebola outbreak continues to grow in Africa; the World Health Organization forecast the death toll would surpass 4,500 by the end of the week.

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, and the United Nations made an urgent appeal for more money to fight the disease.

Canceling a campaign fundraising trip for the second straight day, Obama met into the evening with top aides and health officials. The White House said Obama also placed calls to Canadian Prime Minister Stephen Harper and Swedish Prime Minister Stefan Lofven to discuss the need for an international response to the outbreak in West Africa.

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 allow for longer periods of time. Speaking to reporters at the White House, Obama said several people leading the government’s Ebola response also have other priorities.

“It may make sense for us to have one person ... so that after this initial surge of activity we can have a more regular process just to make sure we are crossing all the Ts and dotting all the Is,” he said.

He said he had no “philosophical objection” to imposing a travel ban on West Africa if it would keep Americans safe but had been told by health and security experts it would be less effective than measures already in place.

Earlier in the day, during a contentious congressional hearing, Republican after Republican demanded that Obama impose a travel ban.

When federal health officials stressed the importance of stopping the outbreak at its source, Rep. Fred Upton, R-Mich., responded, “You’re right, it needs to be solved in Africa. But until it is, we should not be allowing these folks in, period.”

“People’s lives are at stake, and the response so far has been unacceptable,” declared Upton, chairman of the Energy and Commerce Committee. A handful of congressional Democrats also have endorsed the travel ban that’s mainly been pushed by Republicans.