WASHINGTON (AFP) - The second Ebola infection of a health worker in the United States was called unacceptable on Wednesday by a top US doctor, as officials vowed to intensify training of hospital staff.

"What happened there, regardless of the reason, is not acceptable," said Dr Anthony Fauci, head of the National Institute for Allergy and Infectious Diseases. "It is not acceptable that two nurses taking care of a person, because of their exposure... were infected," he said on MSNBC television.

The woman's identity was not revealed by the health authorities, who said she came down with a fever on Tuesday and was isolated at Texas Health Presbyterian Hospital Dallas.

A test to confirm the presence of the virus is being done by the Centres for Disease Control and Prevention (CDC).

"This is a heroic person, a person who has dedicated her life to helping others," said Judge Clay Jenkins.

"Health officials have interviewed the latest patient to quickly identify any contacts or potential exposures, and those people will be monitored," the Health Department said in a statement.

The CDC said the second worker's positive test came from a preliminary analysis conducted overnight and the CDC is carrying out its own test to confirm the result.

"As we have said before, because of our ongoing investigation, it is not unexpected that there would be additional exposures," it said in a statement.

The latest case at Texas Health Presbyterian Hospital Dallas followed the infection announced on Sunday of nurse Nina Pham, who was among more than 70 staff members who helped care for Liberian patient Thomas Eric Duncan. He was believed to have become infected with Ebola in Liberia, and was hospitalized in Dallas on Sept 28. Mr Duncan died of Ebola one week ago.

After Ms Pham's Ebola infection was announced on Sunday, Dr Fauci and the director of the US Centres for Disease Control and Prevention (CDC) Tom Frieden, said that a "breach of protocol" must have been to blame.

Ms Pham was wearing the CDC-outlined protective gear, and officials have yet to identify any lapse in procedures, though experts say the mask, shield, gloves and gown can be difficult to put on and remove safely.

The virus is spread through close contact with the bodily fluids of an infected person, and health care workers are particularly at risk because of the high volume of vomit and diarrhoea that patients often emit in the late stages of the illness.

Dr Fauci said the health authorities were beginning to offer more "proactive training in general - instead of there being passive training, go look at our website or here is a piece of paper - it's going to be very active", to guard against future cases.

"That should not have happened and what the CDC is trying very aggressively to do right now to find out how and what happened and to make sure it doesn't happen again," Dr Fauci said.

On Tuesday, the CDC said it was monitoring 76 health-care workers at the Dallas hospital for signs of Ebola infection.

The hemorrhagic virus has killed more than 4,400 people across West Africa since the beginning of this year, in the world's largest outbreak to date.