A Texas health care worker has become the first person to contract Ebola on American soil, sparking jitters that precautions taken by medical staff could be insufficient.
Top US health officials say a breach of protocol is to blame for the new patient - the second person infected outside Africa and the second diagnosed in the United States.
The news deals a blow to global efforts to stem the epidemic, which has claimed more than 4000 lives, most of them in the West African countries of Guinea, Liberia and Sierra Leone.
"This development is understandably disturbing news for the patient, the patient's family and colleagues and the greater Dallas community," the US Centers for Disease Control and Prevention said on Sunday.
But the CDC insisted any spread into the community surrounding the Dallas hospital "can be prevented with proper public health measures".
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Earlier, agency chief Thomas Frieden said it was clear that "at some point there was a breach in protocol and that breach in protocol resulted in this infection".
The CDC has launched a probe, as well as a hunt for more health care workers who may have been exposed to the virus.
US President Barack Obama said federal authorities should take "immediate additional steps" to make sure hospitals are ready to follow protocols designed for Ebola patients, according to the White House.
The unidentified female carer at Texas Health Presbyterian Hospital in Dallas tested positive on Saturday and is in a stable condition in isolation.
"This is not news that should bring about panic," said Dallas County Judge Clay Jenkins.
But Dan Varga, chief clinical officer for Texas Health Resources, said the woman "was following full CDC precautions".
Frieden said the woman had "extensive contact" with Thomas Eric Duncan, a Liberian diagnosed with Ebola in the United States last month and who died in Texas on Wednesday.
"We are evaluating other potential health care worker exposures," said Frieden. "It is possible that other individuals were exposed."
Meanwhile in Nebraska, a 33-year-old American photojournalist who became infected with Ebola while working in Liberia is continuing to improve, doctors say.
The latest case underlines growing United Nations and US concern 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 the three hard-hit West African countries.
New York's JFK airport on Saturday became the first in the country to launch screenings for travellers from the region.
Passengers at other US hubs will also have their temperatures taken, be screened for signs of illness and questioned about possible exposure.
In Spain, attention remains focused on 44-year-old Teresa Romero, the Madrid nurse who became the first person infected with the hemorrhagic fever outside of Africa.
The Spanish crisis cell set up after she fell sick says there is "reason to hope" she could recover.
Romero is thought to have contracted the disease in late September while caring for a missionary infected with Ebola in Africa who later died.
Romero says she might have brushed her face with a glove - pointing to possible gaps in the safety protocols in hospitals treating Ebola patients.
