U.S. Secretary of State John Kerry on Sunday marked seven years since the disappearance in Iran of American Robert Levinson and urged Tehran to work with Washington to ensure his safe return.

"Robert Levinson disappeared seven years ago from Kish Island, Iran, during a business trip. He is one of the longest held American citizens in history," Kerry said in a statement.

"Mr Levinson's disappearance has been heart-wrenching for his wife and children, who feel his absence especially deeply at the many family milestones missed these past seven years," Kerry said.

"We respectfully ask the government of Iran to work cooperatively with us on the investigation into his disappearance so we can ensure his safe return," the statement said.

"This is the seventh year that Mr Levinson has spent without his family. We remain committed to the hard work ahead to ensure that it's his last."

Levinson, a retired FBI agent, flew to the Iranian resort island of Kish in March 2007.

The U.S. government repeatedly said he was on a business trip when he disappeared, but news reports in recent months have said the CIA had been paying him to gather intelligence.

Iran has said it is not holding Levinson, and denies any knowledge of his whereabouts.

The United States has offered a $1 million reward for information leading to the safe return of Levinson, who turns 66 on Monday.

"Given Mr Levinson's health, age, and length of time in captivity, we mark this anniversary with a special sense of urgency," he said.