Three weeks ago, Iranians celebrated on the streets after a framework nuclear deal with world powers was announced. But on Wednesday in Vienna, the hard work of finalising the accord by a June 30 deadline was set to begin.

Following a marathon of negotiations 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 by strangling its oil exports and financial system.

The accord, if completed and implemented, would draw to a close a crisis that has been steadily and dangerously escalating since Iran's nuclear programme was first revealed some 12 years ago.

It could even potentially push "axis of evil" Iran and the "Great Satan" US towards better relations 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 New York Times op-ed published Monday.

- 'Ways to travel' -

The Vienna talks on Wednesday, which starts the process of drafting the deal, were set to involve the European Union's Helga Schmid, representing the six powers, and Abbas Araghchi, Iran's deputy foreign minister.

Officials from other powers were to join later.

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

US Secretary of State John Kerry said in Lausanne where the April 2 understanding was announced that he had "no illusions about the fact that we still have a ways to travel".

The main problem looks to be the timing of when US and EU economic sanctions related to the nuclear dossier -- UN sanctions and those dealing with other areas are separate matters -- will be lifted.

Iranian President Hassan Rouhani said that he wanted this 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. Kerry said this would take "probably six months to a year".

- Fact sheet -

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

According to a US fact sheet, the deal will include Iran slashing the number of centrifuges -- which can make nuclear fuel but also the core of a bomb by "enriching" uranium -- to 6,104 from 19,000 at present.

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 much less specific, 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 new kinds of nuclear machinery.

"What has been done so far does not guarantee an agreement, nor its contents, nor even that the negotiations will continue to the end," Iran's supreme leader Ayatollah Ali Khamenei said on April 9.