A doctor who treated Ebola patients in West Africa has tested positive for the virus in New York City, officials said.

New York Gov. Andrew Cuomo and New York City Mayor Bill de Blasio, along with other city officials, said at a Thursday evening press conference that preliminary tests have confirmed the first case in the city. The CDC will conduct another test to confirm the official diagnosis.

“We want to state at the outset that there is no reason for New Yorkers to be alarmed,” de Blasio said. 

The New York Times was the first to report that the patient, identified as Dr. Craig Spencer, had tested positive for the virus after he was rushed to Bellevue Hospital in New York Thursday.

Spencer, a 33-year-old emergency room doctor, was working with Doctors Without Borders and returned from Guinea more than a week ago. Officials were contacted after he reported a fever and gastrointestinal symptoms, according to a statement from the commissioner of the New York City Department of Health and Mental Hygiene.

Cuomo said that New York City was "as ready as one can be for this circumstance." He said the city is much more prepared than a Dallas hospital was when the first patient diagnosed with Ebola in the U.S. was admitted there last month. 

"The more facts you know, the less frightening the situation is," Cuomo said.

Cuomo said New York officials spoke earlier in the day with the White House's new Ebola czar Ron Klain, who immediately dispatched a CDC Ebola response team to the city.

City officials say Spencer acknowledged riding multiple subway lines and taking a cab to a Brooklyn bowling alley called "The Gutter" in the past week before he started showing symptoms. He also visited the High Line park and went on a three-mile jog. 

The city's health commissioner, Mary Bassett, said Spencer's fiancé and two friends had been quarantined. The city has also been in contact with an Uber driver who drove Spencer, but said the two did not have direct contact. 

According to a rough timeline provided by city officials, the doctor's symptoms developed Wednesday, prompting him to isolate himself in his apartment.

When he felt worse Thursday, he and his fiancé  made a joint call to authorities to detail his symptoms and his travels. EMTs in full Ebola gear arrived and took him to Bellevue in an ambulance surrounded by police squad cars.

His Harlem apartment was cordoned off, and his fiancé , who was not showing symptoms, was being watched in a quarantine ward at Bellevue. Bellevue Hospital is one of eight hospitals designated by New York Gov. Andrew Cuomo to treat Ebola in the state. 

“As a further precaution, beginning today, the Health Department’s team of disease detectives immediately began to actively trace all of the patient’s contacts to identify anyone who may be at potential risk,” New York City Department of Health and Mental Hygiene commissioner Mary Bassett said in an earlier statement. “The Health Department staff has established protocols to identify, notify and, if necessary, quarantine any contacts of Ebola cases,” she said. 

Spencer works at New York-Presbyterian/Columbia University Medical Center, but had not seen any patients or been to the hospital since his return, the hospital said in a statement.

The doctor is a "dedicated humanitarian" who "went to an area of medical crisis to help a desperately under-served population. He is a committed and responsible physician who always puts his patients first," it said.

The Ebola epidemic in West Africa has killed about 4,800 people. In the United States, the first person diagnosed with the disease was a Liberian man, who fell ill days after arriving in Dallas and later died, becoming the only fatality. Two nurses who treated him were infected and are hospitalized.

Four American aid workers, including three doctors, were infected while working in Africa and were transferred to the U.S. for treatment in recent months. All recovered.

Health care workers are vulnerable because of close contact with patients when they are their sickest and most contagious. In West Africa this year, more than 440 health workers have contracted Ebola and about half have died.

The New York doctor is from Michigan and attended Wayne State University School of Medicine and Columbia's University Mailman School of Public Health.

According to his Facebook page, he left for West Africa via Brussels in mid-September. A photo shows him in full protective gear. He returned to Brussels Oct. 16.

"Off to Guinea with Doctors Without Borders," he wrote. "Please support organizations that are sending support or personnel to West Africa, and help combat one of the worst public health and humanitarian disasters in recent history."

The Associated Press contributed to this report.