WASHINGTON (AFP) -- A Texas health care worker who treated an Ebola victim has tested positive for the deadly tropical fever, dealing a blow to the worldwide battle to stem the outbreak.

More than 4,000 people have died of Ebola in seven countries since the start of the year, according to the World Health Organization, and the epidemic appears to be outpacing efforts to fight it.

If preliminary test results are confirmed, the Texas patient would be the second person diagnosed with the illness and apparently the first to contract it on US soil, a day after US airports began screening travelers from epidemic-hit west Africa.

"We knew a second case could be a reality, and we've been preparing for this possibility," said Dr David Lakey, commissioner of the Texas Department of State Health Services.

"We are broadening our team in Dallas and working with extreme diligence to prevent further spread."

The health care worker from Texas Health Presbyterian Hospital in Dallas reported a low-grade fever Friday night and was isolated and referred for testing, Texas health services said in a statement early Sunday. They did not further identify the worker nor detail how exposure to the virus occurred.

The hospital had treated Thomas Eric Duncan, the Liberian man who died on Wednesday. Duncan was believed to have been infected with Ebola before he left Liberia and boarded a plane to visit family in Texas.

The latest Texas case underlines United Nations fears and growing concerns in the United States about Ebola, for which there is no vaccine or widely available treatment.

"The virus is far ahead of us and every day the situation gets worse," the head of the United Nations' emergency Ebola mission, Anthony Banbury, told UN leaders after a tour of Liberia, Guinea and Sierra Leone, the nations worst affected by the worst-ever outbreak of the disease.

Passengers from the three countries arriving at John F Kennedy International Airport in New York will have their temperatures taken and be screened for signs of illness and answer questions about possible exposure, the US Centers for Disease Control and Prevention said.

"Exit screening might not find every person with Ebola, however, it does not have to be perfect to help reduce the spread of Ebola," the CDC said in a statement. Four other major US airports are to start similar checks next week.

The CDC has predicted the number of cases globally could mount in a worst-case scenario to 1.4 million by January, unless strong measures are taken to contain the disease.

Nurse's condition improves

In Spain, attention remained focused on 44-year-old Teresa Romero, the Madrid nurse who became the first person to get infected with the haemorrhagic fever outside of Africa.

Her condition had "improved in the night. She is conscious and talks from time to time when she is in a good mood," a hospital source told AFP.

Romero's brother confirmed that his sister was improving.

"She no longer has a fever, it appears that while remaining seriously ill she's getting better and moving forward, She's still in a serious but stable condition and this gives us hope," Jose-Ramon Romero told private television channel La Sexta.

Teresa Romero is thought to have contracted the disease in late September in a Madrid hospital while caring for a Spanish missionary infected with Ebola in Africa who later died.

Fifteen other people, mostly hospital staff as well as Romero's husband, are under observation at the Carlos III hospital where Romero is being treated. The hospital said none of them were showing any symptoms.

 'Not all Africa'

The WHO reported 4,033 people have died from Ebola as of October 8 out of a total of 8,399 registered cases in seven countries.

Ebola causes fever, diarrhea, vomiting and in some cases internal and external bleeding. It is spread by contact and the exchange of bodily fluids.

The sharp rise in deaths came as the UN said aid pledges to fight the epidemic have fallen well short of the $1 billion needed.

IMF chief Christine Lagarde pleaded on Saturday for the world to remember that not all of Africa had been hit with Ebola.

With Sierra Leone, Guinea and Liberia already seeing their economies crumble because of the disease, Lagarde emphasized: "We should be very careful not to terrify the planet in respect of the whole of Africa."