An American physician who was hospitalized in the United States after being exposed to the Ebola virus in Sierra Leone was allowed home Tuesday, health officials said.

The doctor, whose identity was not released, had been volunteering in a unit for those suffering from the hemorrhagic fever that has already killed nearly 3,500 people in West Africa since the end of last year.

After suffering a needle-prick injury, he was admitted for observation at the National Institutes of Health Clinical Center in Bethesda, Maryland on September 28.

"The initial hospitalization was characterized by a brief period of fever that was subsequently determined to not be related to infection with Ebola virus," the NIH said in a statement.

"Because he had an exposure and a fever, there was a greater chance he had active infection, thus the reason for isolation."

After the patient's fever abated and he felt well, he was allowed to return home to complete his 21-day observation period.

Ebola is transmitted by close contact with the bodily fluids of an infected person. The virus causes fever, vomiting, diarrhea and sometimes fatal bleeding.

The Ebola epidemic has now infected more than 6,500 people in West Africa and killed nearly half of them, according to the World Health Organization.