The White House renewed its commitment Tuesday to finding a South Florida man who went missing six years ago in Iran and is now believed to be the one of the longest-held Americans in history.

Bob Levinson, a retired FBI agent from Coral Springs, disappeared on March 9, 2007 during a business trip to the Iranian island of Kish, according to a statement released by the White House. The United States believes Levinson, who was working as a private investigator at the time, has been detained in Iran.

Both the White House and Levinson's family say Nov. 26, 2013 marks the day that Levinson becomes the longest-held American hostage, surpassing the 2,454 days journalist Terry Anderson spent in captivity after he was abducted in Lebanon in 1985.

Levinson's case was a topic in recent negotiations between U.S. and Iran aimed at addressing Iran's nuclear program and improving diplomatic ties.

"Our family will soon gather for our seventh Thanksgiving without Bob, and the pain will be almost impossible to bear. Yet, as we endure this terrible nightmare from which we can not wake, we know that we must bear it for Bob, the most extraordinary man we have ever known," a post on the family's website, helpboblevinson.com, reads.

A statement from a White House press secretary called for Levinson to be released to his family and friends, saying officials "we respectfully ask the Government of the Islamic Republic of Iran to assist us in securing Mr. Levinson’s health, welfare, and safe return." It welcomed the help of international partners in the investigation into his disappearance.

White House spokesman Josh Earnest said Tuesday that Obama specifically raised Levinson's case as well as those of U.S. citizens Saeed Abedini and Amir Hekmati, who currently are detained in Iran, during a telephone conversation earlier in the fall with Iranian President Hassan Rouhani.

Abedini is a pastor from Boise, Idaho; Hekmati is a former U.S. Marine whose family lives in Michigan.

"It is our view that all of these Americans should have the opportunity to come home," Earnest told reporters traveling with Obama in Los Angeles.

"The US government has made a respectful request of the Iranian regime during this holiday season to consider on humanitarian grounds releasing these three Americans, or at least releasing the two Americans we know are detained and locating the whereabouts of the third, Mr. Levinson."

There is a $1 million reward for information leading to Levinson's safe return.

The Levinson family has released five photographs of the missing man, which they say were emailed to them in April 2011. The photos showed him with a chain apparently wrapped around his hands, wearing an orange jumpsuit and holding up signs, including one that read, “Help Me.”

Anyone who has information about Levinson should get in touch with the family at helpboblevinson.com. The FBI asks anyone with information on Bob Levinson or his captors to contact it at https://tips.fbi.gov. Information will be kept confidential and can be shared anonymously, the agency says.

"To whoever is holding Bob, I ask again for your mercy. Please let him go to reunite with his family," the family said in the post. "Bob, if somehow you see or hear these words: Stay strong. You have a new grandson, just a month old. We can’t wait for you to meet him. We love you and will never stop working to bring you home safely."

More on NBC6.com: