WASHINGTON - An attendee at last week's Conservative Political Action Conference (CPAC), which US President Donald Trump also attended, has tested positive for COVID-19, the American Conservative Union (ACU) said on Saturday.

The exposure occurred prior to the conference held in National Harbor, US state of Maryland, just south of Washington D.C., said the ACU, a conservative grassroots organization, in a statement.

A New Jersey hospital tested the person, and the Centers for Disease Control and Prevention (CDC) confirmed the positive result, said the statement.

"The individual is under the care of medical professionals in the state of New Jersey, and has been quarantined," it said.

Trump and Vice President Mike Pence spoke at the gathering, which took place from Feb 26 to Feb 29.

Also present at the event were a number of administration and cabinet officials, including Secretary of State Mike Pompeo, Health and Human Services Secretary Alex Azar, and newly-appointed White House chief of staff Mark Meadows.

White House press secretary Stephanie Grisham said in a statement Saturday that the White House is aware of the attendee testing positive for the virus.

"At this time there is no indication that either President Trump or Vice President Pence met with or were in close proximity to the attendee," Grisham said in a statement. "The president's physician and United States Secret Service have been working closely with White House Staff and various agencies to ensure every precaution is taken to keep the First Family and the entire White House Complex safe and healthy."

Grisham also said people should continue to follow the CDC's guidance on travel and public gatherings, "which includes staying home if you feel ill, as well as frequently washing your hands with soap and water."

The news emerged as Washington D.C. and neighboring state of Virginia respectively confirmed first cases of COVID-19 on Saturday.

In a press conference Saturday night, D.C. Mayor Muriel Bowser said a D.C. resident in his 50s exhibited symptoms of a respiratory virus in late February. He was admitted to a hospital in the District on March 5.

The patient had no history of recent international travel, nor had he been exposed to anyone with a confirmed case of COVID-19, according to Bowser.

The mayor said D.C. health authorities are investigating the man's contact with other people before he went to the hospital.

A US Marine assigned to Fort Belvoir in Fairfax county, Virginia, tested positive on Saturday for COVID-19 and is currently being treated at Fort Belvoir Community Hospital, according to a Pentagon spokesman.

"The Marine recently returned from overseas where he was on official business," tweeted Jonathan Rath Hoffman, adding that Secretary of Defense Mark Esper and the White House have been briefed.

As of Saturday night, more than 420 cases of COVID-19 were reported in the United States with 17 deaths, according to real-time data maintained by the Center for Systems Science and Engineering at Johns Hopkins University.