The White House said Thursday that a missing former FBI agent may have died in Iran, acknowledging his family's conclusions, but Tehran said he had left the country years ago.

The family of Bob Levinson, who disappeared in 2007 on a mysterious trip to Iran, said Wednesday that US intelligence had convinced them that he died in Iranian custody but that it was unclear when or how.

President Donald Trump did not confirm Levinson's death when asked by reporters late Wednesday but his national security advisor, Robert O'Brien, went further Thursday.

"Iran must provide a complete accounting of what occurred with Bob Levinson. While the investigation is ongoing, we believe that Bob Levinson may have passed away some time ago," O'Brien said in a statement.

Former FBI agent Bob Levinson is seen in 2007, before his disappearance in Iran, in a family photo
Former FBI agent Bob Levinson is seen in 2007, before his disappearance in Iran, in a family photo WWW.HELPBOBLEVINSON.COM / -

Secretary of State Mike Pompeo, in a cautious separate statement, said: "Only Iran knows for certain what happened to Bob since his abduction more than 13 years ago."

Both Pompeo and O'Brien renewed demands that Iran release several US citizens who remain in jail.

Levinson, who would have turned 72 this month, is one of a number of Americans who have disappeared in arch-enemy Iran, but his case has been among the most perplexing.

The father of seven vanished in March 2007 in Kish, an island that has more lenient visa rules than the rest of Iran, and was said to have been investigating cigarette counterfeiting.

A photograph of the captive Bob Levinson released in January 2013 by his wife, who said the image was nearly two years old
A photograph of the captive Bob Levinson released in January 2013 by his wife, who said the image was nearly two years old LEVINSON FAMILY / -

But The Washington Post reported in 2013 that Levinson, who had retired from the FBI, was working for the CIA and had gone on a rogue mission aimed at gathering intelligence on Iran.

Iran's foreign ministry spokesman Abbas Mousavi implicitly denied the family's claim that Levinson died in Iranian custody.

"Based on credible evidence, (Levinson) left Iran years ago for an unknown destination," Mousavi said in a statement.

He added that Iran had done everything it could to trace what happened after he left but found "no evidence of him being alive."

Mousavi called on Washington to make an official announcement if Levinson's death was confirmed, without "politicizing and exploiting the family's feelings."

Alireza Miryousefi, Iran's chief press officer at the United Nations, had said earlier that Tehran had "no knowledge" of Levinson's whereabouts.

The family has accused Iranian authorities of lying and demanding accountability for Levinson.

Family members have also accused some US officials of doing too little to help Levinson.

The US government has long denied Levinson was employed by it, although the 2013 report by The Washington Post said the CIA paid his wife $2.5 million to accept responsibility for his disappearance.

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