She added that the US had asked the Iranian government to make a humanitarian effort to return him safely to his family.

Iran has repeatedly denied knowing what happened to him.

"As the government of Iran has previously offered its assistance in this matter, we respectfully request the Iranian government to undertake humanitarian efforts to safely return and reunite Bob with his family," Mrs Clinton said.

For several years, the US feared that Mr Levinson, 63, had died. He has diabetes and high blood pressure.

But his family received firm evidence he was alive late last year, the Associated Press news agency reports.

No details of the evidence have been released because officials do not want to hinder rescue efforts, correspondents say.

Mr Levinson's wife, Christine, posted a statement on the family's website on Thursday.

"Our family is tremendously encouraged by the news Bob is alive but remains concerned for his safety and well being," she said.

"Our seven children, our two grandchildren, and I await the day we will be reunited."

Mr Levinson was working as a private investigator on a cigarette smuggling case when he went missing in March 2007. He had retired from the FBI in 1998.

Kish has a reputation for organised crime but is a free-trade zone, so Americans do not need visas to travel there.