WASHINGTON: Donald Trump's younger brother Robert died on Saturday after being hospitalized for an undisclosed illness, the US president said in a statement mourning his loss.

"It is with heavy heart I share that my wonderful brother, Robert, peacefully passed away tonight," Trump said in a White House statement.

"He was not just my brother, he was my best friend. He will be greatly missed, but we will meet again. His memory will live on in my heart forever. Robert, I love you. Rest in peace."

The 74-year-old president had visited his brother on Friday at a hospital in New York, staying for about 45 minutes.

US media reports had said Robert Trump was seriously ill, although there were no details.

Donald Trump, who was stopping in New York on the way to his golf club in nearby Bedminster, New Jersey for the weekend, told reporters Friday only that "he's having a hard time."

He has previously called his brother Robert "wonderful" and said they have "had a great relationship for a long time, from day one."

Media reports said Robert was 71.

Democratic presidential hopeful Joe Biden also sent the president his condolences.

"I know the tremendous pain of losing a loved one - and I know how important family is in moments like these. I hope you know that our prayers are with you all," Biden tweeted.

Born in 1948 and the youngest of the five Trump siblings, Robert Trump worked on Wall Street before joining the family real estate business.