Vienna: Iran and the US announced Saturday a prisoner swap in an apparent gesture of goodwill ahead of the expected entry into force of last year's mammoth nuclear deal between Tehran and six major powers.

The four Iranian-American detainees set to be freed by Iran included Washington Post correspondent Jason Rezaian and Saeed Abedini a pastor from Idaho a senior US official said.

The others are Amir Hekmati and Nosratollah Khosravi-Roodsari Washington said. A fifth American identified as Matthew Trevitick was also to be released as part of a different process.

In exchange Washington said it had granted clemency to seven Iranians six of whom are dual US-Iranian citizens and dropped charges against 14 more.

In Vienna meanwhile US Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif hammered out the final details for the implementation of last July's nuclear deal.

This was ahead of an expected UN nuclear watchdog announcement that Iran has complied with its side of the accord allowing the lifting of sanctions including on Iran's lifeblood oil exports.

Zarif said earlier as he arrived in the Austrian capital that this was a "good day for the world" removing "the shadow of a baseless confrontation" in the Middle East.

The International Atomic Energy Agency (IAEA) report was expected to confirm -- later than widely anticipated -- that Iran has dramatically scaled down its nuclear programme as agreed in the hard-won July 14 deal reached in Vienna.

This combined with ultra-close IAEA inspections extend to at least a year -- from a few months previously -- how long Iran would need to make one nuclear bomb's worth of fissile material.

Iran has always denied wanting nuclear weapons saying its activities are exclusively for peaceful purposes such as power generation.

The IAEA's green light means that a raft of US EU and UN sanctions on the Islamic republic can be lifted allowing oil exports to resume and opening up the 80-million-strong country to business.

The Vienna agreement between Iran and six major powers was sealed after two years of rollercoaster negotiations following the June 2013 election of Iranian President Hassan Rouhani.

The highly complex deal drew a line under a standoff dating back to 2002 marked by failed diplomatic initiatives ever-tighter sanctions defiant nuclear expansion by Iran and threats of military action.

In addition it put Iran and the United States on the road to better relations some 35 years after the Islamic revolution that toppled the US-backed shah and at a particularly explosive time in the Middle East.

- Daggers drawn -

The agreement heralded as US President Barack Obama's biggest major foreign policy triumph has by no means been universally cheered however.

Obama's Republican opponents charge that it fails to do enough to ensure Iran will never get an atomic bomb a complaint shared by Israel Iran's arch foe widely assumed to have nuclear weapons itself.

"The 'Implementation Day' of the nuclear agreement ushers us into a new and dangerous era in which Iran is freed from most of its economic sanctions without having to quit its nuclear programme or provide explanations for its military activities" Israeli Strategic Affairs Minister Gilad Erdan said Saturday.

Sunni Saudi Arabia Iran's other great regional rival is also alarmed at the prospect of warmer US-Iran ties and of predominantly Shiite Iran newly flush with oil revenues increasing its influence.

Already Saudi Arabia and Iran fighting a proxy war in Yemen and key players in the Syrian conflict are at daggers drawn following Riyadh's execution of a Shiite cleric in early January and the subsequent ransacking of the Saudi embassy in Tehran.

Iran's imminent return to the oil market has also contributed to the sharp slide in the price of crude to 12-year lows of under $30 per barrel this week putting Saudi Arabia's public finances under strain.

The lifting of sanctions on Iran is "going to put 500000 barrels per day more on the market" said James Williams of WTRG Economics. Some analysts believe $20 a barrel of oil is on the horizon.

- Bumpy road -

The deal has more than a decade to run which is likely to be a bumpy road experts say not least if more hardline governments emerge in Tehran or Washington.

The two countries are still far from being best friends as witnessed by Iran's recent capture of 10 US sailors in Iranian waters although their improved relations did help ensure their swift release.

Iran violated a UN resolution in October when it test-launched a medium-range missile capable of carrying a nuclear warhead a UN panel of experts concluded in a report in December.

A "snapback" mechanism ensures that many of the sanctions can be swiftly reimposed and a special joint commission is meant to handle any misunderstandings.

"Iran may test the boundaries of the agreement" said Kelsey Davenport of the Arms Control Association.

"At the same time punitive actions for violations on both sides should be proportionate and differentiated from technical missteps which may occur under such a complex agreement" she told AFP.

AFP