The United States granted clemency to seven Iranians and dropped charges against 14 more in exchange for the release of four Americans held in Iran, a US official said Saturday.

"Through a diplomatic channel that was established with the focus of getting our detained US citizens home, we can confirm Iran has released from imprisonment four Americans detained in Iran: Amir Hekmati, Saeed Abedini, Jason Rezaian, and Nosratollah Khosravi-Roodsari," he said.

"We offered clemency to seven Iranians, six of whom are dual US-Iranian citizens, who had been convicted or are pending trial in the United States. The United States also removed any Interpol red notices and dismissed any charges against 14 Iranians for whom it was assessed that extradition requests were unlikely to be successful."

Three of Americans freed by Iran on Sunday landed at Geneva airport in Switzerland around 1700 GMT (2 p.m. ET), a Swiss Foreign Ministry spokesman told Reuters, and were due to head to a military base in Germany.

Reuters correspondents saw the Swiss air force Dassault Falcon passenger jet land carrying the freed men taxi to stop close to a U.S. Air Force plane that had arrived on Saturday evening.

Hekmati is a former US Marine arrested in August 2011, Abedini is a Christian pastor detained since December 2012. Rezaian is a Washington Post reporter detained in July 2014. Little is known about Khosravi-Roodsari.

The US official also said that Iran has promised to help the United States determine the whereabouts of Robert Levinson, a former FBI agent that went missing in Iran in March 2007.

The United States made clear to Iran that the prisoner deal was a "one-time arrangement" and should not be viewed as setting a precedent, a U.S. official said.

The Swiss government also played a role in the release of U.S. prisoners, the official said.