Two lawyers defending Donald Trump on federal charges of illegally retaining classified documents quit the case on Friday and a former aide was charged as well in surprise developments one day after the former US president was indicted.

Trump said on his Truth Social platform that his former military valet, Walt Nauta, had been charged in the case. Nauta went to work at Trump's Mar-a-Lago resort after working in the Trump White House. It was unclear what his charges were.

Nauta's lawyer, Stanley Woodward, declined to comment. A spokesperson for Special Counsel Jack Smith, who is leading the prosecution, could not be reached immediately.

Nauta "served proudly with me in the White House, retired as Senior Chief, and then transitioned into private life as a personal aide," Trump wrote on Truth Social.

In an earlier post, Trump said he would be represented in the case by white collar defence lawyer Todd Blanche, who is representing him in a separate criminal case in Manhattan.

Trump made that announcement after his lawyers John Rowley and Jim Trusty quit the case for reasons that were not immediately clear.

"This morning we tendered our resignations as counsel to President Trump," the two lawyers said in a statement. "It has been an honour to have spent the last year defending him, and we know he will be vindicated."

Trump faces seven criminal counts related to his treatment of sensitive government materials he took with him when he left the White House in January 2021, according to a source familiar with the matter.

He is due to appear in the Florida court on Tuesday, a day before his 77th birthday.

"I AM AN INNOCENT MAN!" Trump wrote on his Truth Social platform on Thursday after announcing he had been indicted. (Reuters)