Federal prosecutors have notified former US President Donald Trump that he is the target of an investigation into his handling of classified material, ABC News reported, fresh legal problems as he fights to return to the White House in 2024.

Politico reported separately that prosecutors sent Trump a letter informing him he was under investigation, but did not clarify the subject of that investigation.

The Justice Department usually notifies people when they become the target of an investigation to give them a chance to present their own evidence to a grand jury. The notification does not necessarily mean Trump will be impeached.

Trump's campaign did not immediately respond to a request for comment, and lawyers were not immediately available for comment.

Trump, the front-runner for the 2024 Republican presidential nomination, has repeatedly called the investigations against him politically motivated.

A federal grand jury is investigating Trump's retention of classified materials after he leaves the White House in 2021.