Six world powers are racing against time to reach a deal with Iran to curb its nuclear programme.
Several issues remain unresolved after 18 months of negotiations between Iran and the United States, Britain, France, Germany, Russia and China. Here's a look at the key issues:
1. Uranium Enrichment
Iran has 19,500 centrifuges which could be used to enrich uranium to the 5 per cent purity needed for nuclear power stations or to the 90 per cent level required to make nuclear weapons. Tehran insists that nuclear power is its only goal.
The US, which had previously demanded that Iran give up every centrifuge, is now willing to let it keep an enrichment capacity, provided it remains at least one year away from being able to make enough weapons-grade uranium for one nuclear bomb.
Securing this 12-month "breakout" time is Washington's bottom line for a deal. The assumption is that it will give the international community enough time to react and stop Iran before it could produce a nuclear bomb.
This is dependent on two variables - the number of centrifuges at Iran's disposal and the amount of low-enriched uranium in its stockpile. The fewer centrifuges and the less uranium, the longer the "breakout" time.
Under a possible compromise, Iran would cut its holding of centrifuges to about 6,000 machines and export its entire stockpile of 7.9 tons of low-enriched uranium, except for a residual amount. This would translate into the 12-month "breakout" time demanded by the US.
The restrictions on centrifuges and uranium would apply for at least 10 years, after which they would expire and Iran would be treated like any other signatory to the Nuclear Non-Proliferation Treaty.
2. Lifting of sanctions
Iran wants all sanctions to be lifted as soon as a final deal is signed. Foreign Minister Mohammad Zarif has said "sanctions, pressure and an agreement will not go together."
But US officials are worried that lifting the sanctions at once could result in Tehran breaking the deal later. Washington wants a step-by-step lifting of sanctions, linked to the steps that Iran would take. It also wants the option of re-imposing sanctions if Iran were to renege on the agreement.
An added complication is there are three layers of sanctions enforced by the US, the European Union and the United Nations.
Provided a final agreement is reached, the EU and the UN should be willing to lift sanctions. The problem lies in the US where the Republican-controlled Congress may oppose any relaxation of the pressure on Iran, particularly if President Barack Obama's critics say he has signed a bad deal.
The question is whether Mr Obama could lift sanctions unilaterally using his presidential powers, or whether this would necessarily require a vote in Congress.
3. Research and Development
Most of Iran's 19,500 centrifuges are of the obsolete "IR-1" variety but it also has three types of more advanced centrifuges which could enrich uranium many times more quickly. Iran wants to keep the option of developing them while any deal is in force. But negotiators are concerned that Tehran may develop a new generation of sophisticated centrifuges that are ready to go into operation the moment the restrictions in an agreement expire. So they want Iran to freeze the research and development of these machines.
4. Inspections
To ensure that Iran does not build a nuclear weapon using a secret network of uranium or plutonium plants, the world powers want Iran to open up its uranium mines and its centrifuge factories to inspection. Iran would also have to implement the "Additional Protocol", an agreement giving UN nuclear inspectors the power to make snap visits. In the past, Iran had signed the Additional Protocol, although its provisions were never implemented.
The International Atomic Energy Agency (IAEA) also has a list of outstanding questions for Iran on its past research into the technology required for nuclear weapons. Revealing its previous efforts to master banned technology could be very embarrassing for Iran. One key issue will be whether a final agreement compels Iran to come clean with the IAEA and address all of the inspectors' concerns.
5. Plutonium
Any deal will have to lay out what nuclear sites Iran would be allowed to maintain. The US does not want Iran to be allowed to develop weapons-grade plutonium at its unfinished Arak reactor. Plutonium can be used as an alternative fissile material to highly-enriched uranium.
Shutting down this possible avenue to a bomb would require Iran to re-engineer the Arak plant. Iran is believed to have agreed in principle to convert this facility into a civilian "Light Water Reactor" with no military use. But whether this conversion would be reversible is another matter.
SOURCE: TELEGRAPH, AFP