The United States on Monday reiterated a call asking for Iran's help in finding a retired agent of the Federal Bureau of Investigation (FBI) and raised the bounty for his whereabouts.

Secretary of State John Kerry's appeal came on the eighth anniversary of Robert Levinson's disappearance, who Washington said went missing on Iran's Kish Island during a business trip.

"We ask the government of the Islamic Republic of Iran to work cooperatively with us on the investigation into Robert Levinson's disappearance so we can ensure his safe return," Kerry said in a statement.

Describing Levinson as "one of the longest held U.S. citizens in history," the top American envoy said "We remain committed to the safe return of Mr. Levinson to his family and appreciate the support and assistance from our international partners."

He announced that the FBI had increased its reward fivefold to 5 million dollars for information leading to Levinson's safe return.

Press reports said Levinson was paid by the U.S. Central Intelligence Agency to gather intelligence inside Iran, while Tehran has repeatedly denied it is holding the American.

Kerry had raised the case of Levinson and other missing Americans directly with Iranian Foreign Minister Mohammad-Javad Zarif "on several occasions" when they met time and again in recent months over Tehran's nuclear program, State Department spokeswoman Jennifer Psaki told reporters Monday.

The other Americans known to be held in Iran are Amir Hekmati, a former Marine, Saeed Abedini, a pastor, and Jason Rezaian, a Washington Post reporter.