A patient being treated at a Dallas hospital has tested positive for Ebola, the first case of the disease to be diagnosed in the United States, federal health officials have announced.

Officials at Texas Health Presbyterian Hospital said the unidentified patient is being kept in isolation and that the hospital is following Centers for Disease Control and Prevention recommendations to keep doctors, staff and patients safe.

The patient is a Liberian national who was admitted on Sunday, a government official told Al Jazeera.

The hospital had announced a day earlier that the patient's symptoms and recent travel indicated a case of Ebola, the virus that has killed more than 3,000 people across West Africa and infected a handful of Americans who have travelled to that region.

Thomas Frieden, director of the CDC, held a news conference at the centre's headquarters in Atlanta late on Tuesday.

“The infected person came from Liberia on September 19 and began to develop symptoms on September 24. He first sought care on the 26th of September and on the 28th was admitted in Texas,” Frieden said.

"Blood samples tested positive for Ebola… The Ebola test is highly accurate," Frieden said, adding: "There is no doubt in my mind that we will stop it here (in the US)."

The CDC has said 12 other people in the US have been tested for Ebola since July 27. Those tests came back negative.

Under observation

Four American aid workers who have become infected while volunteering in West Africa have been treated in special isolation facilities in hospitals in Atlanta and Nebraska, and a US doctor exposed to the virus in Sierra Leone is under observation in a similar facility at the National Institutes of Health.

The US has only four such isolation units but the CDC has insisted that any hospital can safely care for someone with Ebola.

According to the CDC, Ebola symptoms can include fever, muscle pain, vomiting and bleeding, and can appear as long as 21 days after exposure to the virus.

Jason McDonald, spokesman for the CDC, said health officials used two primary guidelines when deciding whether to test a person for the virus.

"The first and foremost determinant is have they traveled to the region (of West Africa)," he said.

The second is whether there has been proximity to family, friends or others who'have been exposed, he said.

US health officials have been preparing since summer in case an individual traveller arrived here unknowingly infected, telling hospitals what infection-control steps to take to prevent the virus from spreading in health facilities.

People boarding planes in the outbreak zone are checked for fever, but symptoms can begin up to 21 days after exposure.

Ebola is not contagious until symptoms begin, and it takes close contact with bodily fluids to spread.