John Bolton (R) was a top advisor to Donald Trump (L) in his first term as president, but later became a fierce critic of his former boss / AFP

John Bolton, who served as Donald Trump's national security advisor before becoming an outspoken critic of the US president, pleaded not guilty on Friday to charges of mishandling classified information.

The 76-year-old veteran diplomat entered the not guilty plea to 18 counts of transmitting and retaining top secret national defense information at a court hearing in Greenbelt, Maryland.

Bolton was released on his own recognizance by Magistrate Judge Timothy Sullivan, who set the next hearing date for November 21.

Bolton, the third Trump foe to be hit with criminal charges in recent weeks, was indicted on Thursday and accused of sharing classified files by email with two "unauthorized individuals" who are not identified but are believed to be his wife and daughter.

The Justice Department said the documents "revealed intelligence about future attacks, foreign adversaries, and foreign policy relations."

Each of the counts carries a maximum sentence of 10 years in prison.

Bolton did not speak to reporters at the Greenbelt courthouse on Friday but he rejected the charges in a statement on Thursday, saying he had "become the latest target in weaponizing the Justice Department."

Bolton's indictment follows the filing of criminal charges by the Trump Justice Department against two other prominent critics of the Republican president -- New York Attorney General Letitia James and former FBI director James Comey.

James, 66, was indicted in Virginia on October 9 on charges of bank fraud and making false statements related to a property she purchased in 2020 in Norfolk, Virginia.

James, who successfully prosecuted Trump for financial fraud, has rejected the charges as "baseless" and "political retribution."

Comey, 64, pleaded not guilty on October 8 to charges of making false statements to Congress and obstructing a congressional proceeding.

His lawyer has said he will seek to have the case thrown out on the grounds it is a vindictive and selective prosecution.

- 'Unfit to be president' -

Trump recently publicly urged Attorney General Pam Bondi in a social media post to take action against James, Comey and others he sees as enemies, in an escalation of his campaign against political opponents.

Trump did not specifically mention Bolton in the Truth Social post, but he has lashed out at his former aide in the past and withdrew his security detail shortly after returning to the White House in January.

Trump called Bolton a "bad guy" on Thursday.

Bolton served as Trump's national security advisor in his first term and later angered the administration with the publication of a highly critical book, "The Room Where It Happened."

He frequently appears on television news shows and in print to condemn the man he has called "unfit to be president."

Since January, Trump has taken a number of punitive measures against perceived enemies, purging government officials he deemed to be disloyal, targeting law firms involved in past cases against him and pulling federal funding from universities.

The cases against James and Comey were filed by Trump's handpicked US attorney, Lindsey Halligan, after the previous federal prosecutor resigned, saying there was not enough evidence to charge them.

Appointed to head the Federal Bureau of Investigation by then-president Barack Obama in 2013, Comey was fired by Trump in 2017 amid the probe into whether any members of the Trump presidential campaign had colluded with Moscow to sway the 2016 election.

cl