Former US President Donald Trump confirmed on Thursday that he has been indicted over his handling of classified documents after leaving office, in another legal threat to 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 Truth Social platform, though there was no immediate confirmation from the Justice Department regarding Trump's assertion.

A Trump attorney, Jim Trusty, told CNN that his client has been indicted on seven charges, including the deliberate retention of documents in violation of the Espionage Act, making false statements, obstruction of justice, and a count of conspiracy.

In his post, Trump said he has been summoned to a federal courthouse in Miami next Tuesday.

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

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

Trump 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.

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

In a video released after he announced the news, Trump repeatedly declared his innocence and considered that the indictment was a form of election interference by a US Justice Department "weaponized" by President Joe Biden's administration.

"They come after me because now we're leading in the polls again by a lot against Biden," Trump claimed in the clip.

"Our country is going to hell and they come after Donald Trump... We can't let this continue," he added.

Speaker of the House of Representatives, Kevin McCarthy, rushed to defend Trump.

"Today is indeed a dark day for the United States of America. It is unconscionable for a President to indict the leading candidate opposing him," McCarthy indicated in a statement.

"I, and every American who believes in the rule of law, stand with President Trump."

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

The FBI transported some 11,000 papers after serving a search warrant on Mar-a-Lago in August, and obstruction-of-justice charges could be a result of his resisting efforts to recover the cache.

Trump eventually turned over 15 boxes containing almost 200 classified documents to the National Archives in January 2022 but was subpoenaed for any outstanding records in his possession.

Trump has repeatedly denied wrongdoing in the case. But he has openly acknowledged taking and storing the documents, undermining his lawyers' suggestion that he took the documents unintentionally.

Read more: New tape leaked of Trump speaking of classified information: CNN