The first case of the deadly Ebola virus diagnosed on US soil has been confirmed in Dallas, Texas.
Officials at Texas Health Presbyterian Hospital say the unidentified patient is being kept in isolation.
The man is thought to have contracted the virus in Liberia before travelling to the US nearly two weeks ago.
More than 3,000 people have already died of Ebola in West Africa and a small number of US aid workers have recovered after being flown to the US.
The US Centers for Disease Control and Prevention (CDC) says the Ebola virus seems to have been contained in Nigeria and Senegal, with no new cases reported there for almost a month.
"An individual travelling from Liberia has been diagnosed with Ebola in the United States," CDC Director Thomas Frieden told reporters on Tuesday.
According to Dr Frieden, the unnamed patient left Liberia on 19 September and arrived in the US the next day to visit relatives, without displaying any symptoms of the virus.
Symptoms of the virus became apparent on 24 September, and on 28 September he was admitted to a Texas hospital and put in isolation.
The disease, which is not contagious until symptoms appear, is spread via close contact with bodily fluids.
Aid workers who caught Ebola in West Africa have come back to the US for treatment but this is the first case of a patient developing the virus on US soil, says the BBC's Alastair Leithead in Los Angeles.
A hospital official told reporters on Tuesday the facility already had procedures in place to deal with any such cases.
Preliminary information indicates that the unnamed patient, who was described as critically ill, was not involved in treating Ebola-infected patients while in Liberia.
Health officials are working to identify all people who came into contact with the unnamed patient while he was infectious, including family and a "couple" community members.
Those people will then be monitored for 21 days to see if an Ebola-related fever develops.
But they will not be monitoring passengers on the man's flight, where Dr Frieden said there was "zero risk of transmission" as the man had been checked for fever before boarding.
'We will stop it'
According to Dr Frieden, it is possible a family member who came in direct contact with the patient may develop Ebola in the coming weeks.
But "the bottom line here is I have no doubt that we will control this importation, this case of Ebola, so it does not spread widely in this country," he added. "We will stop it here."
The World Health Organization (WHO) says more than 3,000 people have died of the virus so far, mostly in Liberia.
Earlier on Tuesday, the head of a new UN body set up to fight the disease urged more action within the next 60 days.
Anthony Banbury told reporters in Ghana that 70% of infected people needed to be receiving treatment and 70% of burials should be done safely within two months.
It is the world's most deadly outbreak of the virus.
Ebola virus disease (EVD)
- Symptoms include high fever, bleeding and central nervous system damage
- Spread by body fluids, such as blood and saliva
- Fatality rate can reach 90% - but current outbreak has mortality rate of about 70%
- Incubation period is two to 21 days
- There is no proven vaccine or cure
- Supportive care such as rehydrating patients who have diarrhoea and vomiting can help recovery
- Fruit bats, a delicacy for some West Africans, are considered to be virus's natural host