Robert O'Brien, national security adviser for U.S. President Donald Trump, attends the 7th ASEAN-U.S. Summit in Bangkok, Thailand, Nov. 4, 2019. (Xinhua/Rachen Sageamsak)

O'Brien reportedly came down with coronavirus after a family event and has been isolating at home while still running the National Security Council (NSC), doing most of his work by phone.

WASHINGTON, July 27  -- Robert O'Brien, national security advisor for U.S. President Donald Trump, has tested positive for COVID-19, the White House announced on Monday.

"He has mild symptoms and has been self-isolating and working from a secure location off site," the White House said in a statement. "There is no risk of exposure to the President or the Vice President."

The diagnosis has made O'Brien, a top aide for Trump, the highest ranking White House official known to have tested positive for the virus, which has infected more than 4.2 million Americans and killed more than 145,000 people in the United States.

Speaking to reporters at the White House before a trip to North Carolina Monday afternoon, Trump said he hasn't seen O'Brien lately.

"I heard he tested. Yeah. I have not seen him. I'm calling him later," said the president, adding that he didn't know when his fourth national security advisor first tested positive.

The White House's announcement on O'Brien came shortly after multiple U.S. media outlets broke the news on Monday morning.

O'Brien, 54, has been out of the office since late last week, Bloomberg News reported, citing people familiar with the situation. He left his office in the White House last Thursday, CNN reported, citing a source.

O'Brien reportedly came down with coronavirus after a family event and has been isolating at home while still running the National Security Council (NSC), doing most of his work by phone.