The latest indictment comes with Trump facing numerous other probes as he bids to be the Republican nominee to challenge President Joe Biden for the presidency in 2024

Donald Trump said on Thursday he has been indicted over his handling of classified documents after leaving office, the US ex-president's most serious legal threat yet as a firestorm of criminal investigations imperil his bid for a second White House term.

"The corrupt Biden Administration has informed my attorneys that I have been Indicted, seemingly over the Boxes Hoax," Trump wrote on his platform Truth Social as he broke what would be bombshell news of a historic moment for the United States: the first time a sitting or former commander-in-chief has ever faced federal charges.

There was no immediate confirmation from the Justice Department regarding Trump's assertion, although some US media outlets including The New York Times cited sources saying that Trump has been indicted.

In his post, Trump, who is running for president again, said he has been summoned to a federal courthouse Tuesday in Miami.

"I never thought it possible that such a thing could happen to a former President of the United States," he wrote.

Trump was charged with seven counts, people familiar with the matter told the Times. They include wilfully retaining national defence secrets, making false statements and an obstruction of justice conspiracy.

Trump, who turns 77 next week, has repeatedly denied any wrongdoing.

His announcement came a day after US media said federal prosecutors have informed the former president's lawyers that he is the target of the probe into his handling of classified documents.

He was already the first former or sitting president to be charged with a crime – in his case over election-eve hush money payments to a porn star who said she had an affair with him. That indictment, handed down by Manhattan's district attorney, came down in March.

In a statement after his initial online posts, the Trump campaign lashed out at what it called an "unprecedented abuse of power" and called for the indictment to be thrown out.

Trump "has long been the biggest threat and the top political target for Joe Biden and the corrupt Democrat Party," his campaign said in a statement.

"Today's act of open legal 'warfare' by the highly politicized and partisan Department of Injustice, has taken things to a new level, and set a dangerous precedent. 

His allies in Congress swiftly rallied to his defence, including congressman Jim Jordan, who chairs the House Judiciary Committee.

"It's a sad day for America. God bless President Trump," he said in a statement.

Special counsel Jack Smith, named by US Attorney General Merrick Garland, has been looking into a cache of classified documents that Trump had stored at his Mar-a-Lago residence in Florida after leaving the White House.

The FBI carted away some 11,000 papers after serving a search warrant on Mar-a-Lago in August, and Trump could face obstruction-of-justice charges after spending months resisting efforts to recover the trove.

Trump eventually turned over 15 boxes containing almost 200 classified documents to the National Archives in January last year but was subpoenaed for any outstanding records in his possession.

Investigators have been scrutinizing several suspected attempts to obstruct them from getting access to documents and footage from a security camera near the storeroom at Mar-a-Lago where documents were kept. 

Mounting legal woes 

Some Democratic lawmakers spoke out following the extraordinary news.

Trump's apparent indictment "is another affirmation of the rule of law," House Democrat Adam Schiff said.

"For four years, he acted like he was above the law," Schiff added. "But he should be treated like any other lawbreaker. And today, he has been."

Trump has repeatedly denied wrongdoing in the documents case, including at a Fox News town hall event on June 1, when he said: "I don't know anything about it. All I know is this: Everything I did was right."

But he has openly acknowledged taking and storing the documents, undermining his lawyers' suggestion that he took the stash inadvertently in the confusion of a chaotic departure.

"This evidence just adds to the mound of stuff that already exists, and no one piece is the 'be all and end all,' but when you put them all together, the case is so strong," former Watergate prosecutor Jill Wine-Banks told MSNBC. 

"You cannot imagine his getting away with this."

The latest indictment comes with Trump facing numerous other probes as he bids to be the Republican nominee to challenge President Joe Biden for the presidency in 2024.

Smith is also looking at whether Trump should face charges over the 2021 US Capitol riot, and Georgia prosecutors are investigating whether Trump illegally attempted to overturn the 2020 presidential election outcome there. 

The former president has already been charged with dozens of financial crimes as part of an alleged hush money scheme to silence a porn star claiming she had sex with him, and is due to go on trial next March, in the middle of primary election season.