WASHINGTON (AFP/REUTERS) - Top US health authorities said on Sunday that a breach of protocol was to blame for the Ebola infection of a Texas health care worker who was said to have worn protective gear.

The exact nature of the breach has not been revealed. After Texas hospital officials announced that the worker had worn a mask, shield and gloves during her encounter with the patient, Thomas Eric Duncan - who died Wednesday - the head of the US Centres for Disease Control and Prevention, Tom Frieden, said he was alarmed by the hospital's failure to report a problem.

"I think the fact that we don't know of a breach in protocol is concerning because clearly there was a breach in protocol," Frieden told CBS Face the Nation. "We have the ability to prevent the spread of Ebola by caring safely for patients."

Emphasising the growing concern in the United States over the arrival of the often-fatal virus, Frieden later held a nationally-televised press conference to confirm the breach and to warn that other health workers may have been infected, although only one person may have had contact with healthcare worker while she may have been infectious.

The woman represents the first case of Ebola infection on US soil, and she is the second person to be diagnosed outside Africa States with the illness that has killed more than 4,000 people in West Africa. She was the first to have contracted the disease in the United States.

The caregiver, whose identity was not revealed by Texas Health Presbyterian Hospital Dallas, tested positive for Ebola on Saturday in a preliminary test and is currently in the hospital, in isolation and in stable condition.

A confirmation of the case was expected later in the day.

A close contact of the health care worker - who CNN said was a female nurse, citing an unnamed official - has also been placed in isolation, officials said at a press conference.

"This is not news that should bring about panic," said Dallas county judge Clay Jenkins.

"We expected that something - that it was possible that a second person could contract the virus." The worker's family has requested total privacy, he added.

But Dan Varga, the chief clinical officer for Texas Health Resources, said officials were "very concerned" that the worker had apparently contracted the virus despite following safety protocol.

"This individual was following full CDC precautions," he said, noting that the gear would have included a mask, gown, and gloves.

Varga said the health care worker came in contact with Duncan the second time he came to the hospital. Duncan arrived in Dallas from Liberia on September 20, and began feeling sick about four days later. He was initially sent home from a Dallas hospital on September 25 when he sought treatment for a fever and abdominal pain, even though he also told staff of his recent travel to Africa. He returned by ambulance on September 28 to the same hospital.

Duncan was estimated to have come in contact with about 100 people, and officials narrowed that pool down to 48 in the days after his infection was diagnosed.

The health care worker was not in the high risk group of people, Varga said, but had been asked to check his or her temperature twice daily and report back if any signs of fever, according to a protocol approved by the CDC.

Anthony Fauci, head of the National Institute for Allergy and Infectious Diseases, echoed Frieden's comments.

"Certainly there has to have been an inadvertent innocent breach of the protocol of taking care of a patient within the personal protective equipment," Fauci said on ABC's This Week.

"That extremely rarely happens. We've been taking care of Ebola patients since 1976. Groups like Doctors without Borders who do that almost never have an infection because of the experience of doing this." He said the CDC is trying to find out what the breach involved.