By Steve Gorman

LOS ANGELES (Reuters) - An American freelance cameraman working for NBC News in Liberia has tested positive for Ebola, the network said on Thursday, making him the fifth citizen of the United States and its first journalist known to have contracted the virus in West Africa.

The 33-year-old cameraman and writer, who has worked in Liberia for the past three years and has covered the recent Ebola outbreak for various U.S. media outlets, will be flown back to the United States for treatment, NBC said in an online report.

Four other NBC News team members who have shown no signs of infection also will return to the United States to undergo a precautionary quarantine, the network said.

Word that a journalist had fallen ill with the potentially lethal virus seemed to raise the stakes for other members of the news media trying to cover the worst Ebola outbreak on record on the ground in Liberia, the nation hardest hit by the epidemic.

The outbreak has killed at least 3,300 people in West Africa.

NBC declined to give the man's name at the request of his family. He began experiencing symptoms on Wednesday that included aches and fatigue, the network said.

He was hired on Tuesday to serve as a second cameraman for NBC News chief medical editor and correspondent Dr. Nancy Snyderman, who has been with three other network employees on assignment in Liberia's capital, Monrovia.

Immediately after beginning to feel sick and discovering he was running a slight fever, the cameraman quarantined himself and sought medical advice. He then went to a Doctors Without Borders treatment center to be tested for the virus, and the positive result came back less than 12 hours later, NBC said.

"We are doing everything we can to get him the best care possible," NBC News President Deborah Turness said in a note to network staff.

Turness also said that as a precaution, Snyderman and the rest of the NBC crew would be flown back to the United States on a private charter plane and will place themselves under quarantine for 21 days, which she said is "at the most conservative end of the spectrum of medical guidance."

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For now, she said, Snyderman and her crew were being closely watched and had shown no symptoms of signs of the illness.

In an interview Thursday with the host of "The Rachel Maddow Show" on NBC's sister cable network, MSNBC, Snyderman said the cameraman's exposure to the potentially lethal virus is believed to have occurred before he began working for the network.

'ZERO RISK MEANS NEVER COMING TO LIBERIA'

But she offered no particulars of how he might have contracted the virus, which is transmitted through contact with the bodily fluids of someone who is infected and symptomatic.

Snyderman said journalists in Liberia carry thermometers for regularly taking their temperatures and observe such precautions as avoiding handshakes and hugs, as well as washing their hands with diluted bleach and water and dipping their feet into bleach solution before entering hotels or other public places.

She said she wore a biohazard suit recently when visiting an Ebola ward, and was helped out of it afterward by two nurses who "meticulously" removed the suit from her body.

"Obviously zero risk means never coming to Liberia," she said.

The four other Americans who have been infected were doctors or relief workers who were sent back to the United States for medical treatment.

Aid workers Dr. Kent Brantly and Nancy Writebol contracted the disease at a relief agency in Monrovia in July. Last month, Dr. Rick Sacra tested positive after working in a Liberian hospital. They have all since been released.

An American doctor diagnosed with Ebola in the neighboring country of Sierra Leone arrived at Emory University Hospital in Atlanta for treatment on Sept. 9 and is still being treated. He has not been publicly identified.

A Liberian man visiting relatives in Dallas recently became the first Ebola patient to be diagnosed in the United States.

(Reporting by Steve Gorman; Editing by Sandra Maler, Peter Cooney and Ken Wills)