Roger Stone has removed the photo from his Instagram account (Reuters)

US President Donald Trump's former adviser Roger Stone apologized on Monday after posting on Instagram a photo of the judge overseeing his trial for allegedly making false statements to Congress, obstruction and witness tampering.

The photo of U.S. District Judge Amy Berman Jackson, which included a symbol that appeared to represent crosshairs, was later taken down.

An attorney for Stone said in a filing in the United States District Court for the District of Columbia that Stone "apologizes to the Court for the improper photograph and comment" and said "Mr. Stone recognizes the impropriety and had it removed."

The judge issued a gag order on Friday that prohibits lawyers involved in the case from speaking with the news media and prohibits other participants, such as Stone himself, from making statements that may affect the case when they are near the courthouse.

"A photo of Judge Jackson posted on my Instagram has been misinterpreted. This was a random photo taken from the Internet," Stone said in a statement posted to Instagram. "Any inference that this was meant to somehow threaten the Judge or disrespect court is categorically false."

Stone has pleaded not guilty to charges of making false statements to Congress, obstruction and witness tampering in Special Counsel Robert Mueller's probe of Russia's role in the 2016 U.S. election and whether President Trump's Republican campaign conspired with Moscow.