Washington (AFP) - While five American citizens were freed from detention in Iran this weekend, the US government vowed Sunday to work tirelessly for the release of another missing for nine years.

Ex-FBI agent Robert Levinson disappeared in mysterious circumstances in March 2007 during a visit to the Iranian island of Kish. He was reportedly investigating cigarette counterfeiting in the region.

Levinson, 67, is considered to be the longest-held hostage in US history, if still alive.

"Even as we rejoice in the safe return of others, we will never forget about Bob," President Barack Obama said in a White House speech.

"Each and every day, but especially today, our hearts are with the Levinson family, and we will not rest until their family is whole again."

Of the US citizens who have been freed by Iran, four were part of a prisoner swap with America, while a fifth was released in a separate process, according to US officials.

Taking to Twitter, Secretary of State John Kerry said "For over a year, we have raised the cases of American citizens unjustly detained in #Iran at every opportunity."

He added: "Iran also agreed to deepen our coordination as we work to locate Robert Levinson. We won't rest until the Levinson family is whole again."

The Federal Bureau of Investigation has announced a $5 million reward for information leading to his return.

"The FBI expects our Iranian counterparts to fulfill their commitment to locate Bob and help bring him home safely," the agency said in a statement.

"Bob forever remains part of the FBI family, and we remain committed to bringing him home safely to the family who misses him so much."

The FBI said "we continue to investigate and follow up on all information we receive, no matter how insignificant it might seem."

In 2013, US media reports revealed that Levinson had been paid by the CIA to gather information during his visits to Iran.

He had been hoping to meet an informant during his trip who could provide information about Tehran's disputed nuclear program, they said.

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The White House has denied Levinson was working for the US government when he vanished.

Washington has repeatedly requested information from Iran concerning Levinson. Iranian officials have denied all knowledge regarding his disappearance.

A father of seven, Levinson suffers from diabetes and hypertension, factors which have raised fears from his family that he may not be receiving proper healthcare.