WASHINGTON -President Donald Trump's national security adviser, Robert O'Brien, has tested positive for COVID-19.

According to CNN, O'Brien's diagnosis marks the highest-ranking Trump administration official known to have tested positive. It's unclear when O'Brien last met with the President. Their most recent public appearance together was over two weeks ago during a visit to US Southern Command in Miami on July 10.

O'Brien, one of Trump's top aides, recently returned from Europe, where he and his top deputy met with officials from the UK, France, Germany and Italy.

A senior administration official told reporters O'Brien has been working from home since last week. A source familiar said O'Brien was last in the office last Thursday, when he abruptly left the White House.

Several National Security Council staffers told reporters they weren't informed that O'Brien tested positive and learned of the news from media reports.

The National Security Council chief was accompanied on the trip to Europe by Secret Service agents, staffers and multiple reporters. Multiple pictures released from the trip showed O'Brien neither practicing social distancing nor wearing a mask.

As the pandemic has spread, the President has defended not wearing a mask or social distancing and a number of top officials and White House staffers have followed his example, saying masks are not necessary because they are tested every day.

Last week, administration officials were alerted that a cafeteria employee on White House grounds had tested positive.

Kimberly Guilfoyle, the President's son's girlfriend and top campaign fundraising official, tested positive before a trip to Mt. Rushmore earlier this month.

Multiple Secret Service agents and campaign staffers tested positive after becoming infected while preparing for the President's Tulsa campaign rally in June.

Vice President Mike Pence's communications aide Katie Miller tested positive for coronavirus in May, and one of the President's personal valets tested positive earlier that week.

O'Brien's positive test is the latest in a series of infection cases that underscore the unique challenges of keeping the virus away from the West Wing and the President's immediate orbit.