Wednesday will be the 2,000th day since Levinson, a former FBI agent, disappeared March 9, 2007.
"Mr. Levinson was last seen on Kish Island and the United States reiterates its call on the government of the Islamic Republic of Iran to provide any information on Mr. Levinson's whereabouts and to help ensure his prompt and safe return to his family," Nuland said in a release.
"Determining Mr. Levinson's whereabouts and reuniting him safely with his family continues to be a priority for the U.S. government. The United States also continues to welcome the assistance of our international partners in this investigation."
A $1 million reward offered by the FBI this spring has failed to produce information leading to Levinson.
Levinson's family issued a statement Tuesday urging anyone with information about the 64-year-old husband, father and grandfather -- who they noted has been held hostage longer than any American in history other than Terry Anderson, the Associated Press Beirut bureau chief who was held in Lebanon for 2,455 days before his freedom was secured.
"This is not a milestone any human being would want to achieve," the family said. "We love and miss Bob more than words can say, and we desperately worry about his health with each day that passes."
The family received a "proof of life" video almost two years ago, "but we have heard nothing from Bob's captors even after making repeated pleas for his release."
"We don't know where Bob is or how he is," the family said. "Once again, we beg Bob's captors to tell us what we need to do to get Bob home. We are beyond ourselves with worry. It is time for him to be released."
The family said Levinson has diabetes, high blood pressure and gout.
Family members previously have said he was going to Iran to investigate cigarette smuggling for a private client. A November 2010 video showed him sitting in a makeshift cell.
"I need the help of the United States government to answer the requests of the group that has held me," he said on the tape. "Please help me get home. Thirty-three years of service to the United States deserves something."
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