Q&A: Iran nuclear crisis
One of the thorniest disputes between Iran and the international community is over Iran's nuclear programme. Here is an explainer to the crisis.
Why is 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 a uranium enrichment plant at Natanz and a heavy-water reactor at Arak.
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.
The US and EU have imposed additional sanctions on Iranian oil exports and banks since 2012, crippling Iran's economy.
Despite this, Iran continues 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 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 in 2013.
What have Iran and the P5+1 agreed?
On 24 November 2013, negotiators reached an interim deal after intensive talks in Geneva. It marked the biggest breakthrough in about a decade of on-off meetings.
Iran agreed to curb its enrichment activities in return for an easing of some sanctions. This "first step" agreement will apply for six months, giving time for a "comprehensive solution" to the crisis to be found.
Under the interim deal, Iran agreed to halt uranium enrichment above the level of 5% purity, reduce its existing stockpile of medium-enriched uranium, not increase the number of centrifuges (used to purify uranium), suspend work at its reactor at Arak (currently under construction), and give UN inspectors more information and greater access to enrichment facilities.
In return, the world powers agreed to "limited, temporary... reversible" sanctions relief. This includes allowing Iran to trade in gold and precious metals, suspending some sanctions on its motor industry, permitting petrochemical exports and giving Iran access to $4.2bn from oil sales. The US says the measures could be worth $7bn in revenue to Iran. They also agreed not to impose any new nuclear-related sanctions for six months.
Critically for Iran, its chief negotiator Foreign Minister Mohammad Javad Zarif said Iran had not given up its right to enrich uranium - one of the sticking points which had held up a deal. The US however denied any such right had been conceded.
What has been the reaction to the deal?
The two sides in the negotiations have hailed the agreement. President Barack Obama said it 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.
Israeli Prime Minister Benjamin Netanyahu said it was "not a historic agreement. It was a historic mistake".
He said Israel was "not bound by this agreement".
Why does the West suspect Iran is seeking nuclear weapons?
Iran's Supreme Leader, 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 level 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 airplane, ship or missile.
Experts at the Washington-based Institute for Science and International Security said in a report that Iran could generate weapons-grade uranium 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.
Doesn't Israel have a nuclear bomb?
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. Israel has had a nuclear research facility in Dimona since the 1960s, and in 1986 former worker Mordechai Vanunu revealed details of a clandestine weapons programme at the plant.
Iran has accused the West of double standards in opposing its nuclear programme, but not Israel's.
However, Israel is not a party to the NPT, so is not obliged to report to it. Neither are India or Pakistan, both of which have developed nuclear weapons. North Korea has left the treaty and has announced that it has acquired a nuclear weapons capacity.
On 18 September 2009, the IAEA called on Israel to join the NPT and open its nuclear facilities to inspection.