Fresh talks aimed at finalising a nuclear deal between Iran and world powers will focus on the lifting of sanctions and possible intervention by US Congress, a top Iranian negotiator said Wednesday.

Iran and the P5+1 group of nations agreed earlier this month to a framework deal aimed at putting a nuclear bomb out of Tehran's reach in return for an easing of economic sanctions.

US President Barack Obama called the interim agreement a "historic understanding" but Iran is concerned that US Congress could introduce a bill to block a final accord, which must be struck by June 30.

"We will ask the American delegation to explain this issue and will ask for clear and precise information on the details regarding the removal of sanctions," deputy foreign minister Abbas Araghchi told the official IRNA news agency.

He warned that intervention by Congress could have "negative consequences" on the nuclear talks, which were set to resume in Vienna on Wednesday.

"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.

If fully implemented, a deal will see 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.

Araghchi, who is leading the Iranian delegation, said that "the lifting of sanctions should not be linked to conditions unrelated" to Tehran's nuclear activities, such as its ballistic missile programme.

Iran, subject to international economic penalties since 2006, wants complete sanctions relief once a deal is struck.

But world powers insist this shall only occur once the nuclear inspectors confirm that Iran is keeping its side of the bargain.

Iranian foreign ministry spokeswoman Marzieh Afkham said on Wednesday that there was a "possibility of changes in the terms" of the framework agreement thrashed out in Lausanne.

"If there were not going to be changes, it would be pointless to continue the negotiations," she told reporters in Tehran.

Final Iran nuclear deal: tough issues still to crack
Vienna (AFP) April 22, 2015 - Negotiators working to a June 30 deadline were on Wednesday set to begin finalising a historic nuclear deal curtailing Iran's nuclear activities in exchange for lifting painful international sanctions.

If completed and implemented successfully, the accord would make any attempt by Iran to make a nuclear weapon -- which it denies wanting to do -- extremely difficult and easily detectable.

Despite Iran and six major powers having agreed on April 2 the main outlines of the accord in Lausanne, Switzerland, there are several potential problem areas to resolve in what will be a highly complex agreement:

- Sanctions -

The United States and European Union have committed to suspending economic sanctions they have imposed on Iran, although officials say that they will "snap back" into place if Iran violates the deal.

All past UN Security Council resolutions on the nuclear issue will be lifted and replaced by a new text endorsing the final deal and incorporating some UN sanctions such as those on conventional arms, missiles and asset freezes.

Iranian President Hassan Rouhani said a week after the Lausanne breakthrough that Iran would not sign a final agreement unless "all economic sanctions are totally lifted on the same day".

But Western officials say that they suspension will only happen once the UN atomic agency confirms Iran has taken the key nuclear-related steps under the deal. US Secretary of State John Kerry has said this would take six to 12 months.

- Numbers, timeframe -

According to a US fact sheet, Iran has committed to slashing the number of centrifuges enriching uranium -- which can render it suitable for power generation but also for a bomb -- to 5,060 from 19,000 at present, and for 10 years.

An additional 1,044 centrifuges will remain at the Fordo facility -- which is built into a mountain -- but for 15 years these will be used for purposes other than uranium enrichment, the US says.

The same document says that Iran will reduce its stockpile of low-enriched uranium -- enough for several bombs if further processed -- from 10,000 kilogrammes (22,000 pounds) to 300 kg, and not to expand it for 15 years.

Iran has however called the fact sheet a "mixture of facts and lies", and an April 2 joint statement by Iranian Foreign Minister Mohammad Javad Zarif and EU foreign policy chief Federica Mogherini was much vaguer.

They said only that "Iran's enrichment capacity, enrichment level and stockpile will be limited for specified durations". This may mean that the exact scope of the downsizing and the timeframe are yet to be nailed down.

The statement did concur however that the Fordo site will no longer be used for enrichment and that a new reactor being built at Arak would be redesigned so that it does not produce weapons-grade plutonium.

- Inspections -

Iran and the six powers also need to work out the details of additional inspection work that the UN atomic watchdog, the International Atomic Energy Agency, would conduct under the final deal.

According to the US, this will include "new transparency and inspections mechanisms", greater access to uranium mines and monitoring a new procurement channel for Iran to acquire nuclear-related materials and technology.

Some of additional oversight measures would be covered by the additional protocol to Iran's existing inspections agreement with the IAEA, as well as the so-called modified code 3.1. The US says Iran will implement both of these.

More oversight is a particular thorny issue in view of allegations that the IAEA wants to investigate that before 2003, and possibly since, Iran conducted research into developing nuclear weapons -- claims that Iran denies.

Western officials stress that these claims of "possible military dimensions" need to be cleared up before sanctions can be lifted, but the IAEA's probe has been stalled since last August.

- Research and development -

A key area of concern is Iran's research into new types of nuclear equipment to replace the 1970s-vintage IR-1 centrifuge machines currently in use.

Critics, including US Republicans and Israel, fear that new machines could enable Iran to make material for a bomb much more quickly, particularly once restrictions on enrichment expire.

According to the US fact sheet, Iran will remove its 1,000 more advanced IR-2M centrifuges and place them under IAEA-monitored storage, while not using more advanced models to enrich uranium for at least 10 years.

Iran will however "engage in limited research and development with its advanced centrifuges, according to a scheduled and parameters which have been agreed to" by the six powers.

US State Department spokeswoman Marie Harf said on April 3 that the two sides "still have some R&D issues to work out, and those are among the most challenging, to be frank."