Former FBI Director James Comey has been charged with threatening the life of US President Donald Trump, a formal accusation that stems from an image he briefly shared on social media.

The image posted on Instagram last year showed seashells forming the numbers "86 47". "Eighty-six" is a slang term used to mean "eject" or "remove".

Comey has insisted he did not know what the numbers on the image meant, but Trump and other administration officials have said the post was a threat against the 47th president.

Responding to the charges on Tuesday, Comey said: "I'm still innocent, I'm still not afraid, and I still believe in the independent federal judiciary."

At a press conference to announce the indictment, FBI Director Kash Patel said that as the former director of the agency, Comey "knew full well the attention and consequences of making such a post".

"James Comey disgracefully encouraged a threat on President Trump's life and posted it on Instagram for the world to see," Kash Patel said on Tuesday.

Comey was fired by Trump during his first term, after the former FBI director opened an investigation into Russian interference in the US 2016 presidential election.

Since then, Trump has repeatedly called for his prosecution - Tuesday's charges are the administration's second attempt to do so.

"Well, they're back. This time about a picture of sea shells on a North Carolina beach a year ago. And this won't be the end of it, but nothing has changed with me," Comey said in a statement.

Comey faces charges of making a threat against the president and transmitting a threat in interstate commerce, according to court documents. Each charge carries a maximum penalty of 10 years in prison.

The criminal charges were filed in the Eastern District of North Carolina, the same region where the shells were reportedly found.

"Threatening the life of the President of the United States is a grave violation of our nation's laws," Acting Attorney General Todd Blanche said in a statement. "The grand jury returned an indictment alleging James Comey did just that."

US Secret Service agents interviewed Comey last May about the seashell photo.

Comey deleted the Instagram post, saying in a follow-up that he "assumed [the sea shells] were a political message".

"I didn't realise some folks associate those numbers with violence," he added. "It never occurred to me but I oppose violence of any kind so I took the post down."

Trump, who has long criticised Comey, said of the post that "a child knows what that meant".

Some legal experts said the indictment appeared insubstantial and raised new questions about the DOJ's efforts to target Trump's perceived political opponents.

"It's very thin," said Michael Gerhardt, a constitutional law expert at the UNC School of Law.

Comey's social media post will likely be viewed by courts as free speech that is protected by the first amendment, Gerhardt added.

Jimmy Gurulé, a former federal prosecutor and former assistant US Attorney General appointed by President George W Bush, said the new indictment was "an embarrassment to the American criminal justice system".

"The DOJ will not be able to prove beyond a reasonable doubt that James Comey had the intent to threaten or harm President Trump," Gurulé, now a Notre Dame Law School professor, said in a statement to the BBC. "The indictment is a transparent attempt to intimidate one of the President's perceived political enemies."

This is the second time the justice department has brought charges against Comey.

Comey was indicted by a federal grand jury in late September on charges that he lied to Congress during testimony in September 2020 and obstructed a congressional proceeding.

The charges came days after Trump called on the country's top law enforcement official to more aggressively investigate his political adversaries, including Comey.

Comey pleaded not guilty during a brief court appearance in October before the case was dismissed in November.

US District Judge Cameron Currie tossed the indictment against Comey because of prosecutor Lindsey Halligan's "invalid" appointment as US attorney.

Halligan, the prosecutor in eastern Virginia who secured the indictments, was not authorised to present the charges to the grand jury, the judge said. Halligan is a former White House aide who had never prosecuted a case before.

The judge, however, left the door open for the government to try again.

Earlier on Tuesday, a separate judge ruled that former federal prosecutor Maurene Comey - James Comey's daughter - can move forward with her case challenging her firing by the Trump administration.