A personal aide to former US president Donald Trump pleaded not guilty on Thursday to federal charges of mishandling classified government documents.

Waltine "Walt" Nauta, 40, a US Navy veteran from Guam, entered the plea at a brief hearing in a US District Court in Miami.

"He enters a plea of not guilty on all counts," said Nauta's lawyer, Stanley Woodward.

Nauta, who was wearing a dark blue suit and a light blue shirt, replied "Yes, your honor" when asked by Chief Magistrate Judge Edwin Torres if he understood why he was in court.

Trump, the frontrunner for the 2024 Republican presidential nomination, pleaded not guilty last month to dozens of criminal counts for mishandling some of the US government's most sensitive secrets and scheming to prevent their return.

Nauta, named as a co-conspirator, is charged with six counts for helping Trump hide documents at the former president's Mar-a-Lago estate in Florida.

According to the indictment, Trump took hundreds of classified government documents in cardboard boxes to Mar-a-Lago after leaving the White House in January 2021.

Trump kept the files — which included records from the Pentagon, CIA, and National Security Agency — unsecured at Mar-a-Lago, which regularly hosted large social events, the indictment said.

The documents were stashed at various locations in Mar-a-Lago including a ballroom, a bathroom, Trump's bedroom, and a storage room, it said.

Nauta is accused of conspiring with the former president to hide the documents and could face up to 20 years in prison if convicted.

Trump faces 31 counts of "willful retention of national defense information" relating to specific documents. A conviction on each count carries up to 10 years in prison.

Other charges facing the 77-year-old Trump include conspiracy to obstruct justice, punishable by up to 20 years in prison, withholding a document or record, which also carries a potential 20-year sentence, and making false statements.

Trump, who was impeached twice over allegations of misconduct while in office and was recently found liable for sexual abuse, has vowed to stay in the 2024 White House race regardless of the outcome of the documents case.

He faces indictment or ongoing legal scrutiny in four criminal probes — in Washington, Florida, Georgia and New York — and could find himself on trial in multiple cases as he campaigns for the presidency.