New York (CNN)[Breaking news update, posted at 9:53 p.m. ET]

After confirming that tests showed a patient tested positive for Ebola, New York Mayor Bill de Blasio said, “We want to state at the outset there is no reason for New Yorkers to be alarmed.”

A Doctors Without Borders physician who recently returned to New York from West Africa has tested positive for the Ebola virus, becoming the first diagnosed case in the city, a law enforcement official briefed on the matter told CNN.

The doctor, identified as Craig Spencer, 33, came back from treating Ebola patients in Guinea about 10 days ago, and developed a fever, nausea, pain and fatigue Wednesday night.

The physician, employed at New York’s Columbia Presbyterian Hospital, has been in isolation at Bellevue Hospital in Manhattan since Thursday morning, the official said.

Doctor tested for Ebola is Craig Spencer

At a news conference Thursday, New York Mayor Bill de Blasio sought to allay public concerns about the spread of the deadly virus, saying that “careful protocols were followed every step of the way” in the city’s handling of the case. The hospitalized doctor has “worked closely” with health officials, the mayor said.

Spencer posted this image to Facebook on September 18 from Brussells, saying \
Spencer posted this image to Facebook on September 18 from Brussells, saying “Off to Guinea with Doctors Without Borders (MSF). 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.”

Photos: The Ebola epidemicPhotos: The Ebola epidemic

NYC doctor being tested for Ebola

Family: Infected nurse is Ebola-free

The doctor exhibited symptoms of the Ebola virus for “a very brief period of time” and had direct contact with “very few people” in New York, de Blasio told reporters.

On his Facebook page, Spencer posted a photo of himself in protective gear. The page indicates he went to Guinea around September 18 and later to Brussels in mid October.

“Off to Guinea with Doctors Without Borders (MSF)” 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.”

In a statement, Columbia Presbyterian Hospital said the doctor was “a dedicated humanitarian” who went to “an area of medical crisis to help a desperately underserved population.”