Rudy Giuliani, U.S. President Donald Trump's lawyer, has tested positive for COVID-19, Trump said in a tweet on Sunday.

Reuters reported citing two sources that Giuliani was at Georgetown University Hospital in Washington, D.C., on Sunday. One of the sources told Reuters he had been admitted. 

It was not clear if Giuliani was experiencing any symptoms. Giuliani's son, Andrew, a special assistant to Trump, tweeted that his father was "resting, getting great care and feeling well." Andrew Giuliani also tested positive for COVID-19 last month.

The president wrote in a tweet: "Get better soon Rudy, we will carry on!"

The former New York City mayor has been going on a cross-country effort to challenge the election results and persuade Republican state lawmakers to help reverse Trump's defeat.

On Sunday evening, he retweeted Trump's announcement tweet.

The 76-year-old former New York mayor has traveled extensively to battleground states in an effort to help Trump subvert his election loss. On numerous occasions he has met with officials for hours at a time without wearing a mask.

Earlier Sunday, he made an appearance on Fox News "Sunday Morning Futures" to speak about his legal challenges in several states on behalf of Trump.

Giuliani did not quarantine after being near an infected person at a November 19 news conference at the Republican National Committee's headquarters. His son Andrew Giuliani announced a day after the event that he had tested positive for the virus.

Research shows that people who contract the virus may become infectious to others several days before they start to feel ill.

Giuliani also appeared maskless at a November 25 hearing in Pennsylvania. Boris Epshteyn, another Trump adviser, tested positive shortly after appearing alongside the lawyer during a press conference on the same day.

Giuliani is the latest person in the president's inner circle to catch the virus. Others include the president's chief of staff Mark Meadows, press secretary Kayleigh McEnany, advisers Hope Hicks and Stephen Miller, along with his wife Melania and sons Donald Jr. and Baron.

The extraordinary spread in Trump's orbit underscores the cavalier approach the president has taken to a virus that has now killed 282,231 people in the U.S. alone. 

(With input from agencies)