Iran and major powers on Wednesday began the difficult process of finalising by June 30 a historic deal putting an Iranian nuclear bomb out of reach, three weeks after agreeing the main outlines.

Following a negotiating marathon in Switzerland, Iran agreed on April 2 to what US President Barack Obama called a "historic understanding... which, if fully implemented, will prevent (Iran) from obtaining a nuclear weapon."

This will include Iran dramatically scaling back its nuclear activities and submitting those that remain to what Obama described the "most robust and intrusive inspections and transparency regime ever negotiated".

In return, the United States and five other major powers committed to lift certain sanctions that have caused the Islamic republic of 75 million people major economic pain.

The accord, if completed and implemented, would draw to a close a crisis that has been raging since Iran's nuclear activities was first revealed some 12 years ago. It denies wanting the bomb.

It could even potentially see "axis of evil" Iran and the "Great Satan" United States bury the hatchet after 35 years of bitter acrimony -- and at a particularly volatile time in the Middle East.

"With courageous leadership and the audacity to make the right decisions, we can and should put this manufactured crisis to rest and move on to much more important work," Iranian Foreign Minister Mohammad Javad Zarif said in a New York Times op-ed published Monday.

- Experts -

The talks in Vienna on Wednesday, starting the process of drafting the deal, involved senior EU diplomat Helga Schmid, representing the P5+1 group, and Abbas Araghchi, Iran's deputy foreign minister, as well as legal and technical experts from all six powers and Tehran.

Other officials including US Under Secretary Wendy Sherman were to join later in the week.

The process of fitting together all the interlocking pieces in what will be a fiendishly complex accord is full of potential pitfalls, experts say.

The main problem looks to be the timing of when US and EU economic sanctions related to the nuclear dossier will be lifted.

Araghchi told the official IRNA news agency Wednesday that he was seeking "clear and precise information on the details" of how this will work.

Iranian President Hassan Rouhani said he wants the removal to occur "on the first day of the implementation of the deal".

But Western officials say this will only happen once the UN atomic watchdog has verified that Iran has taken key steps in the agreement such as removing nuclear machinery. Washington says this would take six months to a year.

The powers will also lift all nuclear-related UN Security Council resolutions and replace them with a new text endorsing the deal and retaining some current UN restrictions such as on ballistic missiles.

The six powers want to retain the ability to "snap back" the sanctions if Iran violates the deal.

- Slashing centrifuge numbers -

The details on other key areas also still have to be nailed down.

According to a US fact sheet, Iran will cut the number of uranium centrifuges -- which can make nuclear fuel but also the core of a bomb -- to 6,104 from 19,000 at present. Around 1,000 of these will not enrich uranium.

In addition, Washington says, Iran will shrink its stockpile of enriched uranium by 98 percent. Taken together this will extend the "breakout" time needed to make one bomb's worth of material to at least one year.

Iran has however criticised the fact sheet and a joint statement by Zarif and EU foreign policy head Federica Mogherini on April 2 was vague, saying only that Iran's enrichment capacity and stockpile would be "limited".

Other areas that still have to be cleared up include the details of the IAEA's expanded inspections role and the future scope of Iran's research and development of more advanced types of nuclear machinery.

Further complicating matters is the opposition to the mooted deal among hardliners in Iran and Washington, where Republicans -- like Israel -- worry the deal is too weak. Gulf countries including Saudi Arabia are also uneasy.

Araghchi said that he also wanted the US side to "explain" the implications of the approval by a US Senate panel last week of a measure giving Congress input on the final deal, warning this could have "negative consequences".

"The US is part of multilateral negotiations and it is the responsibility of this government to ensure that its obligations, in particular those related to sanctions, will be implemented in full," Araghchi said.

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New York Times