The departure of three American prisoners from Tehran, released as part of a swap following the lifting of sanctions on Iran was at least partly delayed by a "temporary misunderstanding" about whether two relatives were on the plane.

There had been expectations that they would leave on Saturday, while the final round of talks on sanctions were taking place. But the Swiss plane carrying Jason Rezaian, the Washington Post's Tehran bureau chief, Saeed Abedini, a pastor from Idaho and Amir Hekmati, a former Marine from Flint, Michigan as well as some of their family members did not leave until Sunday morning.

It had been reported when the plane took off that Nosratollah Khosravi-Roodsari, about whom little is known, was on board. But a senior U.S. official later said he was not traveling with the other released prisoners.

It was not immediately clear whether he opted to stay in Iran or depart separately or whether this uncertainty had in any way contributed to the delay. An earlier State Department statement said "those who wished to depart Iran have left."

A fifth prisoner, American student Matthew Trevithick, left Iran on Saturday under a separate arrangement.

U.S. Secretary of State John Kerry told reporters after landing in Washington early on Sunday from talks in Vienna that the delay in the departure of the prisoners was in part due to a "temporary misunderstanding" about whether Rezaian's mother, Mary, and his wife, Yeganeh Salehi, also a journalist, were on the plane, as had been agreed. They were later confirmed as being on the plane.

Mary Rezaian was in Tehran pressing for her son's release.

Kerry said he raised the issue with Iranian Foreign Minister Javad Zarif in Vienna during final talks on Iran's compliance under the nuclear deal, a step that was necessary for the lifting of U.S. and other sanctions against Tehran.

"I told Javad (Zarif), look she's on the list and is part of the agreement. He said absolutely, and he assigned four people to it and got onto it immediately," said Kerry. "Before I left last night and got onto the plane, we had complete clarity that she was going to leave, no issue, it just was a glitch. These things happen."

Kerry said the release of the prisoners followed months of meetings with Zarif and between other senior U.S. officials and Iranian officials on the sidelines of nuclear talks, which began in earnest in 2013.

"It was very complicated because there were lots of different pieces at stake and moving parts," said Kerry, referring to months of dual-track negotiations with Iran on its nuclear program, while also trying to secure the release of the prisoners.

During nearly a dozen meetings to discuss a possible prisoner swap, Kerry said he urged their release on humanitarian grounds although he worried that the prisoner release could be complicated by Iranian politics.

"I constantly reminded him (Zarif)...look I'm really uncomfortable sitting here negotiating ... these are people, this is a humanitarian effort, and we believe they have been convicted wrongly," recalled Kerry, adding:

"I knew we were going to get there sometime but I couldn't tell if it was going to get super-complicated by (another agency) and political dynamics into the mix."

Kerry said the whereabouts of Robert Levinson, a former FBI agent and DEA agent, who disappeared in Iran since 2007, was also part of the prisoner agreement with the Iranians.

U.S. officials have believed for several years that Levinson died in captivity. Iranian officials had repeatedly denied any knowledge of his disappearance or whereabouts.

"He is specifically mentioned by name in the agreement and there is an agreement to specifically cooperate to find him," said Kerry. "We have gone to other countries, we have involved other people, there have been all kinds of clues that have come in and they have even followed up on," Kerry said.

"With the cooperation of the Iranians we are making our very best effort to get information regarding him."

By Reuters