The United States for the first time accused Iran of direct involvement in the "probable death" of former FBI agent Bob Levinson, who vanished 13 years ago, and imposed sanctions on two intelligence agents.

Releasing the finding a month before President-elect Joe Biden takes office, Donald Trump's administration urged his successor to prioritize the release of at least three Americans in Iranian custody as his team expects to resume diplomacy with the US adversary.

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

"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 are largely symbolic as Iranian agents are unlikely to have bank accounts in the United States, 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 missing Americans. 

"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 a 2015 agreement negotiated by his predecessor Barack Obama under which Iran dramatically scaled back its nuclear program. 

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