By Ellen Wulfhorst

NEW YORK (Reuters) - A physician with Doctors without Borders who returned from West Africa recently and developed potential symptoms was being tested for Ebola at a New York City hospital, health officials said on Thursday, setting off fresh fears about the spread of the virus.

The doctor was identified as Craig Spencer, who was working for the humanitarian organization in Guinea, one of three West African nations hardest hit by Ebola.

Spencer, 33, developed a fever and gastrointestinal symptoms and notified Doctors Without Borders on Thursday morning, the organization said in a statement.

City health officials were alerted, and Spencer was transported from his Manhattan apartment by a specially trained team wearing protective gear, the New York City Department of Health and Mental Hygiene said in a statement.

Mayor Bill de Blasio said test results on the doctor would be made public, possibly late on Thursday evening.

"It is our understanding very few people were in direct contact with him," de Blasio said at a news conference. "Every protocol has been followed."

The health department said it was tracing all of the patient's contacts to identify anyone who may be at potential risk.

Spencer was being treated at Bellevue Hospital, the health department said. The historic city hospital is one of the eight in New York state designated this month as part of an Ebola preparedness plan.

Spencer's Facebook page, which included a photo of him clad in protective gear, said he went to Guinea around Sept. 18 and then flew to Brussels on Oct. 16.

He has specialized in international emergency medicine at Columbia University-New York Presbyterian Hospital in New York City since 2011, according to his profile on the LinkedIn career website.

Columbia in a statement said he has not been to work nor seen any patients since his return. It called him "a dedicated humanitarian ... who went to an area of medical crisis to help a desperately underserved population."

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APARTMENT SEALED OFF

Americans' fears about Ebola, which has killed nearly 4,900 people, largely in Liberia, Sierra Leone and Guinea, have mounted since the first person diagnosed with it in the United States, a Liberian man who had flown to Texas, was hospitalized in Dallas last month.

The man, Thomas Eric Duncan, died on Oct. 8, and two nurses who treated him became infected with the virus. A task force has been set up following missteps in handling the case.

Before Spencer was admitted between 1 p.m. and 2 p.m. local time to Bellevue, the staff spent about three hours shutting off certain areas to isolate the seventh-floor ward where he would be kept, one nurse aide said.

"Anyone would be scared to be next door to a patient. But if we do it the right way there's nothing to be scared of," said the aide, Kirk Elphage.

A woman at the Bellevue information desk identified herself to a staff member as the patient's fianceé. She appeared very agitated and declined to comment.

Spencer's apartment in Manhattan's Harlem neighborhood was sealed off but the rest of the six-story brick building remained open to residents, health officials said.

Three police officers stood outside. Neighbors stopped to ask what was going on but did not appear visibly concerned.

Another employee at Bellevue said the hospital's staff had been trained and was well prepared for the possible case.

"Everybody's calm, said Maria Delgado, 60, a clerk with the radiology department. "To be quite honest, you really don't know who walks in there anyway."

News of the latest potential Ebola case in the United States caused stocks to pare gains late in the trading session.

"It threw a little scare into the market," said Michael James, managing director of equity trading at Wedbush Securities in Los Angeles. [.N]

The virus is spread through direct contact with bodily fluids from an infected person and is not airborne.

The United States this week began requiring travelers coming from Liberia, Sierra Leone and Guinea to enter through one of five airports conducting increased screening for the virus. It also is directing those travelers to check in with health officials every day and report their temperatures and any Ebola symptoms for 21 days.

(Additional reporting by Barbara Goldberg, Sebastien Malo, Jonathan Allen and Laila Kearney; Editing by Sandra Maler, Jonathan Oatis and Lisa Shumaker)