Q&A: Iran nuclear crisis
Iran and six world powers are in the final stage of negotiations to end a decade-old crisis over Iran's nuclear programme. There are still obstacles to a deal though, compounded by years of distrust.Why is there a crisis?
In short, because world powers suspect Iran has not been honest about its nuclear programme and is seeking the ability to build a nuclear bomb.
Iran says it has the right to nuclear energy - and stresses that its nuclear programme is for peaceful purposes only.Why has it gone on for so long?
Iran's nuclear programme became public in 2002, when an opposition group revealed secret activity including construction of a uranium enrichment plant at Natanz and a heavy-water reactor at Arak. Enriched uranium can be used to make nuclear weapons, and spent fuel from a heavy-water reactor contains plutonium suitable for a bomb.
The Iranian government subsequently agreed to inspections by the global nuclear watchdog, the International Atomic Energy Agency (IAEA).
But the IAEA was unable to confirm Iran's assertions that its nuclear programme was exclusively for peaceful purposes and that it had not sought to develop nuclear weapons.
This led the US and its European allies to press Iran to stop enriching uranium, which can be used for civilian purposes but also - if the concentration of the active uranium-235 isotope is 90% or above - to build nuclear bombs.
However, the election of President Mahmoud Ahmadinejad in 2005 halted any progress in talks, and the IAEA referred Iran to the UN Security Council for failing to comply with its Nuclear Non-Proliferation Treaty (NPT) Safeguards Agreement.
Since then, the Security Council has adopted six resolutions requiring Iran to stop enriching uranium, some imposing sanctions.
In 2012, the US and EU began imposing additional sanctions on Iranian oil exports and banks, crippling Iran's economy.
Despite that, Iran continued to enrich uranium. In 2009, it disclosed the existence of a new underground facility at Fordo.
There have been multiple rounds of negotiations between Iran and the so-called P5+1, which comprises the five permanent members of the UN Security Council - the US, UK, France, China, Russia - and Germany.
For years they failed to make headway. But the mood changed after the election of Hassan Rouhani as president in June 2013. Five months later, following secret bilateral talks between the US and Iran, negotiators agreed an interim deal.What did both sides agree to do?
Under the Joint Plan of Action (JPOA), which came into force in January 2014, Iran effectively agreed to freeze its production of enriched uranium above 5% purity and committed to dilute or convert into oxide form its stockpile of near-20% enriched uranium. Iran said it would not install any new uranium centrifuges or build new enrichment facilities. Iran also agreed not to commission or fuel the reactor at Arak, and not to construct a fuel reprocessing facility there.
While most of the international sanctions regime - including measures targeting Iran's key oil, banking and financial sectors - was kept in place, the P5+1 agreed to permit Iran to repatriate about $4.2bn (£2.6bn) in revenue from oil sales locked in foreign accounts. Sales of petrochemicals and trading in gold and other precious metals were allowed to resume, along with transactions with foreign firms involved in the Iranian car industry. That was expected to generate about $1.5bn in revenue. Iran was also given access to about $400m of cash to pay for tuition for Iranians studying abroad, spare parts for civilian aircraft, and for humanitarian purchases of food and medicine.Will there be a final agreement in November?
Diplomats warned from the start of the negotiations on a comprehensive nuclear accord that they would be complex and that significant gaps remained between Iran and the P5+1. Both sides said the six rounds of talks between January and July 2014 had produced significant progress, including redesigning the Arak reactor to substantially reduce the amount of plutonium it will produce. But they also accused each other of making some unrealistic demands, primarily on the issue of uranium enrichment.
Iran is reportedly offering to freeze the current number of operating centrifuges for three to seven years. After that, it argues, there must be sufficient enrichment capacity to produce fuel for the Bushehr power plant when the reactor's fuel supply agreement with Russia expires in 2021. That would require Iran to expand its current capacity 10-fold or more, which experts say would reduce the amount of time required to produce weapons-grade uranium to a few weeks. In return for higher enrichment capacity and being permitted to continue research and development, Iran says it would accept more intrusive inspections by the IAEA.
The P5+1 wants a sharp reduction of Iran's current enrichment capacity. It says Iran has no compelling need to produce large amounts of fuel because Russia is prepared to supply Bushehr's reactor for its lifetime. The P5+1 also wants to limit Iran's R&D activities, which could enhance centrifuge efficiency. It believes the restrictions should remain in place for at least two decades and be backed-up by extensive monitoring.
Domestic political constraints are said to be limiting the chance of compromise on enrichment. Analysts say the Iranian negotiators would struggle to defend an agreement that does not preserve their country's current capacity. Western negotiators would likewise have difficulty selling a deal that left Iran the ability to rapidly produce weapons-grade uranium.Why is Iran suspected of seeking nuclear weapons?
Iran's Supreme Leader, Ayatollah Ali Khamenei declared in 2009: "We fundamentally reject nuclear weapons and prohibit the use and production of nuclear weapons."
But an IAEA published a report in 2011 claiming "credible" information that Iran had carried out activities "relevant to the development of a nuclear explosive device".
The report drew attention to a military complex at Parchin, south of Tehran, which the IAEA has been unable to visit since 2005.
Reports surfaced in 2000 that a large containment vessel had been built there to conduct hydrodynamic experiments. The IAEA said such experiments, which involve using explosives in conjunction with nuclear material or surrogates, were "strong indicators of possible weapon development".
The US has alleged that Iran had a nuclear weapons programme in 2003, but that senior Iranian leaders stopped it when it was discovered.
In September 2014, the IAEA said Iran had failed to explain unresolved issues over its research into detonators that could be used to trigger a nuclear weapon, and also to explain studies that could be relevant to calculating the explosive yield of a nuclear weapon. Iran was also continuing to refuse to allow inspectors visit Parchin, the IAEA reported.How does Iran justify its refusal to obey UN resolutions?
The technology used to enrich uranium to the level needed for nuclear power can also be used to enrich it to the higher concentration - 90%, or "weapons-grade" - needed for a nuclear explosion.
Iran hid an enrichment programme for 18 years, so the Security Council says that until Iran's peaceful intentions can be fully established, it should stop enrichment and other nuclear activities.
Iran has said it is simply doing what it is allowed to do under the Nuclear Non-Proliferation Treaty, which allows signatory states to enrich uranium to be used as fuel for power generation.
Such states have to remain under inspection by the IAEA. Iran is under inspection, though not under the strictest rules allowed because it will not agree to them.Could Iran build a nuclear bomb if it chose to?
There are mixed views on this. US Secretary of State John Kerry told a Senate hearing in April 2014 that Iran had the ability to produce weapons-grade uranium for a nuclear bomb within two months, if it decided to do so. However, Mr Kerry said such a "break-out" window did not mean Iran yet had a warhead or suitable delivery system. That was also before the IAEA confirmed in July that Iran had converted all of its 20%-enriched uranium into forms that were less of a proliferation risk.
Experts at the Washington-based Institute for Science and International Security cautioned in a September 2013 report that any bomb-making process would be done in secret, so estimating timelines was extremely difficult.
In fact, experts have been predicting for decades that Iran was on the verge of building a nuclear bomb. While the IAEA can monitor the amount of uranium, the relative skills of its scientists involved in nuclear and weapons research are harder to assess.