Raising fresh concern around the world, a nurse in Spain on Monday became the first person known to catch Ebola outside the outbreak zone in West Africa. In the U.S., President Barack Obama said the government was considering ordering more careful screening of airline passengers arriving from the region.

In dealing with potential Ebola cases, Obama said, "we don't have a lot of margin for error."

Already hospitalized in the U.S., a critically ill Liberian man, Thomas Duncan, began receiving an experimental drug in Dallas. But there were encouraging signs for an American video journalist who returned from Liberia for treatment. Ashoka Mukpo, 33, was able to walk off the plane before being loaded on a stretcher and taken to an ambulance, and his father said his symptoms of fever and nausea appeared mild.

"It was really wonderful to see his face," said Dr. Mitchell Levy, who talked to his son over a video chat system at Nebraska Medical Center.

In Spain, the stricken nurse had been part of a team that treated two missionaries flown home to Spain after becoming infected with Ebola in West Africa. The nurse's only symptom was a fever, but the infection was confirmed by two tests, Spanish health officials said. She was being treated in isolation, while authorities drew up a list of people she had had contact with.

Medical workers in Texas were among Americans waiting to find out whether they had been infected by Duncan, the African traveller.

No blanket ban on travel

In Washington, the White House continued to rule out any blanket ban on travel from West Africa.

People leaving the outbreak zone are checked for fevers before they're allowed to board airplanes, but the disease's incubation period is 21 days and symptoms could arise later.

Airline crews and border agents already watch for obviously sick passengers, and in a high-level meeting at the White House, officials discussed potential options for screening passengers when they arrive in the U.S. as well.

Spain Ebola

The stricken Spanish nurse had been part of a team that treated priest Manuel Garcia Viejo (pictured). (San Juan de Dios Order/Associated Press)

Obama said the U.S. will be "working on protocols to do additional passenger screening both at the source and here in the United States." He did not outline any details or offer a timeline for when new measures might begin.

Additional screening would not have caught Duncan because he wasn't exhibiting any Ebola symptoms when he arrived in the U.S.

The Obama administration maintains that the best way to protect Americans is to end the outbreak in Africa. To that end, the U.S. military was working Monday on the first of 17 promised medical centres in Liberia and training up to 4,000 soldiers this week to help with the Ebola crisis.

The U.S. is equipped to stop any further cases that reach this country, said White House spokesman Josh Earnest.

"The tragedy of this situation is that Ebola is rapidly spreading among populations in West African who don't have that kind of medical infrastructure," Earnest said.

About 350 U.S. troops are already in Liberia, the Pentagon said, to begin building a 25-bed field hospital for medical workers infected with Ebola. A torrential rain delayed the start of the job on Monday.

Hundreds of dead health workers

The virus has taken an especially devastating toll on health care workers, sickening or killing more than 370 in the hardest-hit countries of Liberia, Guinea and Sierra Leone — places that already were short on doctors and nurses before Ebola.

Texas Gov. Rick Perry urged the U.S. government to begin screening air passengers arriving from Ebola-affected nations, including taking their temperatures.

Perry stopped short, however, of joining some conservatives who have backed bans on travel from those countries.

Federal health officials say a travel ban could make the desperate situation worse in the afflicted countries, and White House spokesman Earnest said it was not currently under consideration.

Southwest Airlines CEO Gary Kelly said he saw no need for additional screening at airports and noted that airlines already carefully clean planes.

Airlines have dealt with previous epidemics, such as the 2003 outbreak in Asia of SARS, or severe acute respiratory syndrome.

"Now it's Ebola," Kelly said. "We are always on the alert for any kind of infectious disease."

The U.S. didn't ban flights or impose extra screening on passengers during the SARS outbreak or the 2009 swine flu pandemic. Both of those were airborne diseases that spread more easily than the Ebola virus, which is spread by contact with bodily fluids.

The CDC did meet many direct flights arriving from SARS-affected countries, to distribute health notices advising travellers that they might have been exposed, how they could monitor their health and when to call a doctor.