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 reported that 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."

While far less famous than his older brother, Robert Trump, who was born in 1948, had long been an integral part of the family real estate empire and was fiercely loyal to the president.

Robert Trump unsuccessfully went to court to try to get an injunction preventing publication of a book by his niece Mary Trump, called "Too Much and Never Enough: How My Family Created the World's Most Dangerous Man."

The book, which paints Donald Trump as the product of a "toxic" family was a "disgrace," Robert Trump said. Although a temporary restraining order was applied, a judge lifted this in July, allowing publication.

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

Donald Trump's daughter and adviser Ivanka said on Twitter Saturday: "Uncle Robert, we love you. You are in our hearts and prayers, always."