The US on December 14 for the first time accused Iran of direct involvement in the “probable death” of former FBI agent Bob Levinson, who vanished 13 years ago.

Releasing the finding a month before President-elect Joe Biden takes office, Donald Trump’s administration urged his successor to prioritise the release of at least three US citizens in Iranian custody as part of an expected resumption of diplomacy.

FBI Director Christopher Wray said in a statement: “The government of Iran pledged to provide assistance in bringing Bob Levinson home, but it has never followed through.

“The truth is that Iranian intelligence officers – with the approval of senior Iranian officials – were involved in Bob’s abduction and detention.”

The treasury department announced that it was imposing sanctions on two Iranians identified as intelligence agents, Mohammad Baseri and Ahmad Khazai, saying they “were involved in the abduction, detention and probable death of Mr Levinson”.

The sanctions in themselves were largely symbolic as Iranian agents were unlikely to have bank accounts in the US, although the move will impede their international movements.

A senior US official, speaking to reporters on condition of anonymity, called on the incoming Biden administration to address the question of missing US citizens.

“There should be no agreement negotiated with Iran ever again that doesn’t free the Americans who are unjustly detained in that country,” the official said, saying that Iran’s clerical regime “is 41 years old and has a 41-year-old record of hostage-taking”.

Trump has imposed sweeping sanctions on Iran, including trying to stop all of its oil exports, and exited an agreement negotiated by his predecessor Barack Obama under which Iran dramatically scaled back its nuclear programme.

Upon sealing of the 2015 accord, Iran agreed to free four US citizens in its custody. The deal outraged members of Trump’s Republican Party because Obama also unfroze Iranian assets.