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.

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.

Jason Rezaian in 'good spirits' after Iran ordeal: editors
Washington (AFP) Jan 18, 2016 - Jason Rezaian, the Washington Post journalist released by Iran over the weekend after 1.5 years in the country's notorious Evin prison, told his editors Sunday he feels "a hell of a lot better."

Rezaian is among four US citizens freed by Iran in a prisoner swap, and arrived in Germany on Sunday, a US official said.

A fifth American, Matthew Trevithick, was released in a separate process, according to a US official.

"Jason was in good spirits," his editors said in a memo to newsroom staff following brief telephone conversations with Rezaian.

"Asked how he was doing, he said, 'I'm a hell of a lot better than I was 48 hours ago.' He said that he feels better than he did several months ago and that his mind is sharp."

Rezaian's family reported that his health deteriorated in prison, where he lost weight and suffered from high blood pressure, and that he was held for months in isolation without access to a lawyer.

The Post also said he was subjected to physical mistreatment and psychological abuse before finally being convicted of espionage in what it called a sham trial.

"Isolation, as you might expect, was the most difficult thing. When told, well, you're a social person, he laughed and responded, 'Yes, I am!'" Post executive editor Martin Baron and foreign editor Douglas Jehl said.

"He found escape in the fiction he was allowed to read, and today he was avidly reading whatever he wanted... remarking on how strange it was to see himself being talked about so much," they added.

"We told him we've been talking about him for 545 days."

Above all, the editors said, Rezaian "wanted to express his deep appreciation for the strong and unwavering support from everyone at The Post."

"Jason said he'd been able to read some of the coverage of his release on his mom's iPad while on the plane to Germany," Baron and Jehl added.

"The support of the Post 'means everything,' he said."

And Rezaian expressed hope he could thank his colleagues personally in the newsroom in Washington as early as Monday.

"I hear there's going to be a big party," the editors quoted him as saying.

They added: "For now, we agreed, he just needed to get some sleep."

Rezaian, a dual US-Iranian citizen born in California, was detained in Iran on July 22, 2014.

He was arrested at the same time as his wife Yeganeh Salehi, an Iranian who at that time was a journalist for The National, an Abu Dhabi-based newspaper. She was released on bail a few months later.

Both Rezaian, 39, and his wife left Iran on a plane earlier Sunday.

The Post's editors said the reporter had become a hostage to the nuclear talks, being detained at the behest of hardline opponents of President Hassan Rouhani within Iran's regime.

The nuclear deal was finally implemented on Saturday, with sanctions on Iran being lifted.