A Swiss plane carrying American prisoners released by Iran has landed in Geneva, a US official said on Sunday.

"Overwhelmed to greet Jason #Rezaian, his wife, Yegi, mother, Mary + Saeed #Abedini + Amir #Hekmati to #Geneva after safely departing #Iran," senior US diplomat Brett McGurk wrote on Twitter.

Moments earlier, an AFP reporter at Geneva airport saw a Swiss air force plane touch down on the same tarmac where a large, official US aircraft was stationed.

The Swiss foreign ministry earlier announced that three of the four US citizens freed by Iran in a prisoner swap after more than a year of secret negotiations on Swiss soil were due to arrive in Switzerland.

Upon arrival, the freed prisoners would then leave for an American base in Germany, it said in a statement.

The Washington Post said that among those on board was its Tehran correspondent Jason Rezaian, who had been detained in Iran for nearly 18 months.

The other two were understood to be Saeed Abedini, a Christian pastor, and former US Marine Amir Hekmati.

The fourth Iranian-American released as part of the swap, Nosratollah Khosravi-Roodsari, has not yet left Tehran, senior US officials said.

Speaking on condition of anonymity, officials said Nosratollah Khosravi-Roodsari -- about whom little is known -- elected not to board the plane that flew the three others to Switzerland.

"Those who wished to depart Iran have left," said one official. Another said "it's his free determination where he wants to go."

Iran had announced the prisoner releases on Saturday, just hours before Tehran's historic nuclear deal with world powers was implemented, in exchange for Washington pardoning seven Iranians accused of sanctions-busting or violating trade embargoes.