A New York doctor who recently returned from Ebola-hit Guinea in West 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.
Meanwhile, one million doses of an Ebola vaccine will be produced by the end of 2015, it has been announced.
The World Health Organization said "several hundred thousand" will be produced in the first half of the year.
Mali has confirmed its first case of Ebola after a two-year-old girl tested positive for the virus.
More than 4,800 people have died of Ebola - mainly in Liberia, Guinea and Sierra Leone - since March.
EU leaders pledged at a summit on Friday to boost aid to combat Ebola in West Africa to 1bn euros ($1.25bn; £785m), EU president Herman Van Rompuy tweeted.
Member states and the European Commission have already pledged nearly 600m euros.
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.
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. They added that those who came into contact with Dr Spencer were not at risk.
"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."
Ebola in New York: Media reaction
Alexandra Sifferlin writing for Time magazine asks if New York is ready for Ebola. Leading medical experts in the city tell her that they are and add they have learned from mistakes made in Dallas.
In the New York Daily News, a leading city doctor, Judith Aberg, insists Bellevue Hospital is up to the task: "They have been training for this event, as we have at all the major hospitals... the people taking care of him are highly trained and the most fit to be in that room with him."
Abby Haglage in the Daily Beast says Dr Spencer's case could affect Ebola patients worldwide - by deterring would-be volunteers from travelling to West Africa to help.
Julia Ioffe, senior editor at the New Republic, questions why Dr Spencer felt the need to go bowling - especially when he decided to stay home from work. She adds that Amber Vinson, who also had Ebola, took two flights despite running a temperature.
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 fiance and two friends have been placed into quarantine, said Dr Mary Bassett, New York's health commissioner.
Dr Spencer is the fourth person to be diagnosed with the disease in the US.
The first caught Ebola in his native Liberia and travelled to Dallas, Texas, before his symptoms set in. He died on 8 October.
Two nurses who treated him in Dallas subsequently came down with the disease and are recovering in hospital.
Meanwhile, on Thursday the West African country of Mali confirmed its first Ebola case - a two-year-old girl recently returned from Guinea.
The girl's mother died in Guinea a few weeks ago and the child was then brought by relatives to Mali, Reuters news agency quoted a health ministry official as saying.
Mali is now the sixth West African country to be affected by the latest Ebola outbreak - however Senegal and Nigeria have since been declared virus-free by the WHO.
Separately, the World Health Organization (WHO) has already identified at least two experimental vaccines which it believes could be promising.
At a meeting in Geneva, the UN health body said it wanted tests of the vaccines to be completed by the end of December.
The WHO says 443 health workers have contracted Ebola, of whom 244 have died.