Long-time foes Iran and the United States agreed to swap a total of 11 prisoners, both sides announced Saturday as they held last-minute negotiations on implementing an international nuclear deal that will remove sanctions against Iran.

One of the four Iranian-US citizens freed by Iran was Washington Post reporter Jason Rezaian, taken into custody in July 2014 and convicted on espionage charges last year, US and Iranian officials said.

The other three included US Marines veteran Amir Hekmati, a pastor named Saeed Abedini and a man named Nosratollah Khosrawi.

In exchange, the US pardoned seven Iranian citizens who had been convicted or charged in relation to violating sanctions provisions. The US also dropped charges against 14 additional Iranians, a US official told reporters.

The prisoner exchange was announced the same day that US Secretary of State John Kerry, Iranian Foreign Minister Mohammad Javad Zarif and EU chief diplomat Federica Mogherini met in Vienna to clarify remaining issues before the implementation of a broad nuclear deal with six major powers that will end sanctions against Tehran.

Despite the announced prisoner swap, issues had yet to be resolved related to documents submitted at the last minute by Iran.

"The devil is in the detail," a diplomatic source said.

Iran finalized an agreement with the United States, Britain, France, Germany, Russia and China in July to curb Tehran's nuclear programme and prevent it from making nuclear weapons in return for sanctions relief.

The involved countries were still waiting Saturday evening for a final report by the Vienna-based International Atomic Energy Agency (IAEA) to confirm that Tehran has fulfilled the required nuclear steps.

According the July 14 deal, this confirmation will oblige the EU and US to immediately lift sanctions that have been cutting off Iran from global commerce, including an embargo on Iranian oil exports and on imports of airplanes.

Transportation Minister Abbas Akhondi announced Saturday that Iran had ordered 114 planes from the European aviation conglomerate Airbus.

The aircraft are due to be delivered after the implementation of the nuclear deal and the lifting of sanctions, he told Iranian media.

Iran had agreed in July that it would dismantle thousands of uranium enrichment centrifuges, reduce its stocks of enriched uranium from 12 tons to hundreds of kilograms and to pour concrete into its plutonium-producing Arak reactor.

The downsizing of Iran's civilian nuclear programme, along with intrusive IAEA inspections in the coming years, are meant to ensure that Iran cannot make nuclear weapons.

The implementation of the nuclear deal was set to come 13 years after revelations that Tehran had secretly built nuclear facilities and had dabbled in nuclear weapons research.

Iran's regional foes Israel and Saudi Arabia are worried that the nuclear agreement could boost Iran's clout in the region, and that thawing ties between long-time enemies Washington and Tehran could decrease their own status.

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