(This March 7 story has been refiled to drop name in third paragraph)

By Phil Stewart

WASHINGTON (Reuters) - A U.S. Marine in the Washington D.C.-area who recently returned from official travel overseas has tested positive for the coronavirus, the Pentagon said on Saturday, in what would be the third confirmed case among U.S. servicemembers and the first in the United States.

The other two U.S. troops who have tested positive are a sailor in Italy and a soldier in South Korea.

Pentagon spokesman Jonathan Hoffman said in a statement that the Marine was assigned to Fort Belvoir, Virginia, and was currently being treated at Fort Belvoir Community Hospital.

Fort Belvoir is a major base in suburban Virginia, not far from the Pentagon. The U.S. military did not disclose the Marine's rank or say where the Marine had traveled.

Hoffman said Defense Secretary Mark Esper and the White House had been briefed.

A U.S. defense official, speaking on condition of anonymity, said the Marine was a man and that his family was now on a 14-day quarantine. The U.S. military was working with state and local officials to trace contacts that the Marine had in the area.

It was unclear whether Fort Belvoir was taking any other precautionary measures. The base did not respond to a request for comment.

The latest cases illustrate the risks for the military, which is responsible for bases around the world where U.S. troops live and work, often in close proximity to each other. The bases are often like small cities and their ability to remain operational is key to U.S. military readiness.

Last week, Esper said U.S. officials were reviewing plans on how to keep the Pentagon operating in the event of a coronavirus outbreak in one of the world's largest office buildings, where roughly 22,000 people work everyday.

Earlier on Saturday, the U.S. military said a U.S. sailor deployed to Naval Support Activity Naples in Italy had also tested positive for the coronavirus. That led to a broader quarantine on base, but how big remained unclear.

"Personnel that the service member immediately identified having close contact with have been notified and are in self-isolation at their residence," the military said.

U.S. Naval Support Activity Naples is host to more than 50 separate commands and approximately 8,500 personnel and is located approximately 60 miles (96 kilometers) south from the city of Gaeta and 145 miles (235 kilometers) from Rome.

(Reporting by Phil Stewart, Editing by Nick Zieminski)