WASHINGTON (AFP) -- A former FBI agent who mysteriously vanished in 2007 died in Iranian custody, his family concluded Wednesday, saying US intelligence had made them give up 13 years of hope.

President Donald Trump did not confirm Bob Levinson's death, saying that Iran had not communicated any news on the former agent, who would have turned 72 this month.

But Levinson's family said that it had learned that he was dead, although it gave no information on how or when.

"We recently received information from US officials that has led both them and us to conclude that our wonderful husband and father died while in Iranian custody," the family said in a statement.

"We don't even know when, or even if, his body would be returned to us.

This is the very definition of cruelty."

The family said Levinson died before the coronavirus pandemic, which has hit Iran hard and led authorities to release thousands of prisoners temporarily.

Levinson is one of a number of Americans who have disappeared in arch-enemy Iran, but his case has been among the most perplexing, with his family until now insisting he was alive.

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.

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.

It said at the time that the CIA paid $2.5 million to Levinson's wife Christine, accepting responsibility for his disappearance.

His family on Wednesday accused Tehran of "repeatedly lying to the world"  about Levinson and renewed criticism of the initial US response. Officials initially did not classify Levinson as a hostage and have consistently denied he worked for the government.

"Those who are responsible for what happened to Bob Levinson, including those in the US government who for many years repeatedly left him behind, will ultimately receive justice for what they have done," the family said.

"We will spend the rest of our lives making sure of this, and the Iranian regime must know we will not be going away."

Years of mystery

Iranian officials have repeatedly said they had no information about Levinson.

In 2010, a videotape of a haggard, bearded Levinson emerged in which he wore an orange jumpsuit of the sort worn by prisoners being held indefinitely at the US naval base in Guantanamo Bay, Cuba.

The footage raised speculation that he may have been held by extremists in Pakistan, but US officials later discounted the possibility.

Trump, who as a candidate had accused his predecessor Barack Obama of not doing enough for Levinson in negotiations with Iran, noted that the former agent had longstanding health problems.

"I don't accept that he's dead," Trump told reporters when asked about the family's statement.

"They haven't told us that he's dead, but a lot of people are thinking that that is the case," he said.

Senator Bob Menendez, who employs one of Levinson's sons on his staff, sounded more definitive on his death and vowed to hold Iran accountable.

"To be clear, the Iranian regime is fully responsible for Bob Levinson's disappearance and his death," said Menendez, the top Democrat on the Senate Foreign Relations Committee.

Republican Senator Marco Rubio of Florida, where Levinson lived, also called for the United States to "hold the evil regime accountable."

Iran released four US citizens of Iranian descent in 2016 when it concluded a nuclear deal with Obama. but relations have sharply deteriorated under Trump, who withdrew from the accord and implemented sweeping sanctions.

Amid the coronavirus pandemic, Iran last week freed a US Navy veteran, Michael White, who had apparently come to Iran to see a woman he met online and was jailed for online criticism of the regime.

He was handed over to Switzerland, which represents US interests in Iran, but not allowed to leave the country.

Iran is still holding dual US nationals Siamak Namazi, who was convicted of charges that include espionage, his father Baquer and environmental expert Morad Tahbaz. (AFP)