WASHINGTON — Five years after a retired FBI agent disappeared while on a trip to Iran, the head of the US law enforcement agency offered a $1 million reward for information leading to the safe return of Bob Levinson.

Secretary of State Hillary Clinton urged Iran to help with the search and welcomed "the assistance of our international partners in this investigation."

"The US government is committed to Mr. Levinson's safe return and we will continue to use all available resources until he is home and reunited with his family," Clinton said in a statement.

"We also call on the government of Iran to uphold its promise of assistance and help safely return Mr. Levinson to the United States."

Levinson flew to Kish Island, Iran, on March 8, 2007 to look into cigarette counterfeiting while working as a private investigator for a major corporation. He disappeared the following day.

Iran's elite military force, the Revolutionary Guards, has denied reports that it was holding Levinson and the foreign ministry has said it would aid in the search for "humanitarian" reasons.

US officials believe Levinson is being held in southwest Asia -- likely in the border areas of Afghanistan, Iran, and Pakistan -- and is launching a publicity campaign there using billboards, radio messages and flyers to publicize the reward.

"Though he is retired from the FBI, Bob remains a member of the FBI family to this day," FBI director Robert Mueller said at a press conference on the steps of the agency's Washington headquarters.

"His family is our family," Mueller said, as he was joined by Levinson's wife and flanked by dozens of agents.

"We in the FBI will continue to do all that we can to ensure Bob's safe return to Christine and their family, to his FBI family, and to the country that he has served so well and so diligently for 28 years."

A proof-of-life video was sent to his family in 2010 in which Levinson appeared weary and thin but unharmed. It was the first substantial evidence that he is alive and being held against his will.

"I have been treated well, but I need the help of the United States government to answer the requests of the group that has held me for three and a half years," Levinson said in the 54-second clip in which he is shown seated in front of what appears to be a grey concrete wall.

"I am not in very good health. I am running very quickly out of diabetes medicine," he said, his voice quavering. "Please help me get home. Thirty-three years of service to the United States deserves something."

FBI officials examined the video for clues, including Levinson's remarks that a "group" was holding him hostage, suggesting it may be a terror network or crime cartel rather than a government.

A former US State Department official familiar with the case said last year that the video was accompanied by a demand for the release of several US-held prisoners.

"We hope this reward will encourage anyone with information, no matter how insignificant they may think it is, to come forward," said James McJunkin, assistant director in charge of the FBI's Washington field office.

"It may be the clue that we need to locate Bob."

The FBI is currently stumped.

"We have no information about who are the captors, who has him and where he is physically located," McJunkin told reporters, adding there have been "no demands made" and the lack of progress is "very frustrating."

Levinson's family insisted they would never give up hope.

"Our goal is to get Bob home. We miss him every single day," said Christine Levinson, his wife of 37 years.

Levinson, who marks his 64th birthday on Saturday, has seven children and two grandchildren and his lengthy absence has weighed heavily on the family.

There is "no word to describe the nightmare" his family has been living since Levinson's disappearance, she told reporters.

"Our youngest son is about to graduate from high school," Christine Levinson added in a statement. "He was in middle school when his father disappeared."