Washington DC - 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 a 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.

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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.