US President Donald Trump left Washington on Wednesday just three hours ahead of the inauguration of his successor Joe Biden, saying it had been an "incredible four years" and promising to be "back some form."

"I just want to say goodbye but hopefully it's not a long-term goodbye," Trump said in remarks to reporters at the White House before he and First Lady Melania Trump walked a short red carpet and boarded Marine One for the helicopter ride to Joint Base Andrews.

Speaking to a small crowd of supporters, staff and family members at Andrews after a 21-gun salute and a serenade of "Hail to the Chief," Trump said "this has been an incredible four years."

"We've accomplished so much together," he said. "I will always fight for you. 

"It is my greatest honor and privilege to have been your president," he said. "We will be back in some form."

The 74-year-old Trump has hinted at running for president again in 2024, but faces a trial in the Senate over his role in the January 6 storming of the US Capitol by his supporters and a potential ban on holding office in the future.

Trump boarded Air Force One after his remarks and the presidential jet took off for the two-hour flight to Florida as Frank Sinatra's "My Way" played on loudspeakers on the tarmac on a sunny but chilly day.

Trump will be at his Mar-a-Lago resort in Florida when Biden is sworn in at noon (1700 GMT) as the 46th president of the United States.

He is the first president in more than 150 years to snub the inauguration of his successor.

Trump did not address Biden by name in his remarks but said he wishes the new administration "great luck and great success."

A White House spokesman said Trump, in keeping with tradition, had left a letter for Biden in the Oval Office.

Trump repeatedly and falsely claimed that he won the November 3 election and he was impeached for a second time by the House of Representatives after his supporters stormed the Capitol in a bid to disrupt the certification by Congress of Biden's victory.