US President Donald Trump's younger brother Robert Trump dies "It is with heavy heart I share that my wonderful brother, Robert, peacefully passed away tonight," Mr Trump said in a 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."

Mr Trump visited his brother at a New York City hospital on Friday (local time) after White House officials said he had become seriously ill.

He said his brother was "having a hard time" with an undisclosed illness, shortly after the visit.

The cause of Robert Trump's death has not been revealed.

Robert Trump began his career on Wall Street working in corporate finance but later joined the family business, managing real estate holdings as a top executive in the Trump Organisation.

Donald Trump once described his younger brother as "much quieter and easy going than I am", and "the only guy in my life whom I ever call 'honey'."

Robert Trump was born in 1948, the youngest of New York City real estate developer Fred Trump's five children.

A Boston University graduate, Robert Trump managed the Brooklyn portion of his father's real estate empire, which was eventually sold.

Once a regular name in Manhattan's social pages, he had kept a lower profile in recent years.

Before divorcing his first wife, Blaine Trump, more than a decade ago, he had been active on Manhattan's Upper East Side charity circuit.

He avoided the limelight during his elder brother's presidency, having retired to the Hudson Valley.

But he described himself as a big supporter of his brother's White House run in a 2016 interview with the New York Post.

"I support Donald 1,000 per cent," Robert Trump said.

In March this year he married his long-time girlfriend, Ann Marie Pallan.

Two of Donald Trump's children paid tribute to their uncle on twitter.

"Robert Trump was an incredible man — strong, kind and loyal to the core. Anyone who encountered him felt his warmth immediately. He will be deeply missed by our entire family," Eric Trump tweeted.

"Uncle Robert, we love you. You are in our hearts and prayers, always," Ivanka Trump tweeted.