Former FBI director James Comey is being interviewed by the US Secret Service after he shared then deleted a social media post that Republicans alleged was an incitement to violence against US President Donald Trump.

Comey voluntarily participated in the questioning at the law enforcement agency's Washington DC headquarters and was not being held in custody, sources told US media.

It comes a day after he posted on Instagram a photo of seashells that spelled the numbers "8647".

The number 86 is a slang term whose definitions include "to reject" or "to get rid of", however, it has more recently been used as a term to mean "kill". Trump is the 47th US president.

Comey, whom Trump fired as FBI director in 2017, was expected to be interviewed on Friday afternoon about what he meant by the post, and whether he intended it as a threat against Trump or an attempt to inspire others to commit violence, according to CNN.

Trump said earlier in the day during an interview with Fox News that Comey was calling for him to be killed.

"He knew exactly what that meant," said Trump, who survived two attempts on his life last year. "A child knows what that meant.

"If you're the FBI director and you don't know what that meant, that meant assassination. And it says it loud and clear."

Trump said any decision on whether charges should be filed against Comey would be up to Attorney General Pam Bondi.

Comey posted the seashell photo on Thursday then deleted it amid conservative uproar.

He wrote in a follow-up message on Instagram that he had seen the shells during a walk on the beach, "which I assumed were a political message".

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

Director of National Intelligence Tulsi Gabbard, the highest-ranking spymaster in the US, called for Comey to be jailed for "issuing a hit" on Trump while he was travelling in the Middle East.

Secret Service spokesman Anthony Guglielmi said in a statement on Thursday: "We are aware of the social media posts by the former FBI Director and we take rhetoric like this very seriously."