A New York doctor who recently returned from Ebola-hit Guinea inWest Africa has tested positive for the disease.
Dr Craig Spencer, who treated Ebola patients while working for the charity Medecins Sans Frontieres (MSF), came down with a fever on Thursday, days after his return, officials say.
He is the first Ebola case diagnosed in New York, and the fourth in the US.
More than 4,800 people have died of Ebola - mainly in Liberia, Guinea and Sierra Leone - since March.
Dr Spencer, 33, left Guinea on 14 October, and returned to New York City on 17 October via Europe. On Tuesday he began to feel tired and developed a fever and diarrhoea on Thursday.
He immediately contacted medical services and was taken to the city's Bellevue Hospital, where he is being kept in isolation.
President Barack Obama said his thoughts and prayers were with the patient.
Four contacts New York officials said Dr Spencer had travelled on the subway and gone out jogging before he started feeling unwell.
But at a news conference late on Thursday, they sought to ease fears of an outbreak in the densely populated city of 8.4 million people, saying officials had prepared for weeks for an Ebola case.
"There is no reason for New Yorkers to be alarmed," Mayor Bill de Blasio said. "Ebola is an extremely hard disease to contract. New Yorkers who have not been exposed to an infected person's bodily fluids are not at risk."
Governor Andrew Cuomo said, "We can't say that this is an unexpected circumstance."
Mr Obama telephoned both the mayor and the governor to discuss the deployment of health officials and to offer "any additional federal support necessary", the White House said.
Ebola patients are only infectious if they have symptoms, and the disease is only transmittable through bodily fluids, experts say.
Mr Cuomo said officials had identified four people with whom Dr Spencer had contact during the period in which he was potentially infectious.
His fiancee and two friends have been placed into quarantine, said Dr Mary Bassett, New York's health commissioner.