Q&A: Iran nuclear crisis
Iran has begun halting some of the most controversial elements of its nuclear programme as part of a landmark deal struck with world powers to end a crisis that has lasted more than a decade.Why was there a crisis?
In short, because world powers suspect Iran has not been honest about its nuclear programme and is seeking 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.What led to the crisis?
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 UN 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 nuclear purposes but also - if enriched to 90% purity - 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 - the five UN Security Council permanent members, the US, UK, France, China, Russia and Germany. For years the two sides failed to make headway. But the mood changed after the election of Hassan Rouhani as president and on 24 November 2013, negotiators reached an interim deal after intensive talks in Geneva.What has Iran agreed to do?
A summary of the deal published by the White House states that by or on the first day of implementation - 20 January - the IAEA was to confirm that Iran was halting production of near-20% enriched uranium; starting to dilute half of the near-20% enriched uranium stockpile in hexafluoride form; and converting the rest to oxide form not suitable for further enrichment.
Over the remaining six months, the IAEA will verify through daily inspections that Iran is not enriching uranium in roughly half of installed centrifuges at Natanz and three-quarters of installed centrifuges at Fordo; limiting centrifuge production to those needed to replace damaged machines; and not constructing additional enrichment facilities.
Iran has also committed not to commission or fuel the Arak reactor; halt the production and additional testing of fuel for the facility; not to install any additional reactor components there; not to transfer fuel and heavy water to the reactor site; and not to construct a reprocessing facility, preventing the separation of plutonium from spent fuel.
At the end of the six-month period, Tehran will agree to "a cap on the permitted size of Iran's up-to-5% enriched uranium stockpile", the White House summary says. In November, the White House said Iran had committed to halt all enrichment above 5% and dismantle the technical connections required to enrich above that level.What have the P5+1 and EU committed to do?
The P5+1 and EU will provide "limited, temporary, and targeted relief to Iran". The White House summary estimates the total value of the relief at between $6bn (£3.65bn) and $7bn (£4.26bn). However, the most of the sanctions regime - including measures targeting Iran's key oil, banking, and financial sectors - will remain in place.
On the first day of implementation, the P5+1 and EU committed to suspend sanctions on Iran's petrochemical exports; imports of goods and services for its automotive manufacturing sector; and its import and export of gold and other precious metals. They will also license the supply of spare parts and services for Iran's civil aviation sector; help establish a financial channel to support humanitarian trade and facilitate payments for UN obligations and tuition payments for students studying abroad; and modify EU procedures for the authorisation of financial transactions.
The P5+1 and EU have also committed to facilitate Iran's access to $4.2bn in restricted funds on a set schedule at regular intervals throughout the six-month period.What was the reaction to the deal?
President Barack Obama said the interim deal would "cut off Iran's most likely paths to a [nuclear] bomb", while his Secretary of State John Kerry said the agreement would make the region safer for its allies, including Israel.
President Rouhani also welcomed the deal, saying "No matter what interpretations are given, Iran's right to enrichment has been recognised". Iran's Supreme Leader Ayatollah Khamenei, who has final say in nuclear matters, called it an "achievement" and a "success".
The deal however has been sharply criticised by Israel, which sees Iran's nuclear programme as a potential threat to its existence. Prime Minister Benjamin Netanyahu said it was "a historic mistake" and warned that Israel was "not bound" by it. Israel neither confirms nor denies it has nuclear weapons - a policy known as "nuclear ambiguity" - though it is widely believed to possess up to 400 warheads.Why is Iran suspected of seeking nuclear weapons?
Ayatollah Khamenei, who is reported to have issued a fatwa against nuclear weapons, declared in 2009: "We fundamentally reject nuclear weapons and prohibit the use and production of nuclear weapons."
But the 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.How does Iran justify its refusal to obey the Security Council 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?
US President Barack Obama told Israeli television in March 2013 that his administration believed it would take "over a year or so for Iran to actually develop a nuclear weapon" if it decided to do so.
That would mean producing enough weapons-grade uranium; fashioning it into a warhead; and being able to deliver it by aeroplane or missile.
Experts at the Washington-based Institute for Science and International Security said in a report that Iran could turn its 20%-, or medium-enriched uranium into weapons-grade in a matter of months. But the institute cautioned that the bomb-making process is separate and 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 UN can monitor the amount of uranium, the relative skills of its scientists involved in nuclear and weapons research are harder to assess.