Iran and world powers have extended negotiations to end a decade-old crisis over the Iranian nuclear programme. There are still obstacles to a deal though, compounded by years of distrust.
Why is there a crisis?
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.
Diplomats have been seeking a deal that would allow Iran to have nuclear power but reduce the likelihood of it gaining nuclear weapons.
How close are they to a deal?
Iran and the so-called P5+1 - the US, UK, France, China and Russia plus Germany - agreed an interim accord in November 2013 that saw the Islamic Republic curb sensitive nuclear activities in return for relief from sanctions that have crippled its economy.
A deadline of July 2014 was set to find a "comprehensive solution", but the talks were subsequently extended by four months to give negotiators more time.
Despite "significant progress" being made, both sides agreed in November that another seven months were needed. Western diplomats are confident of securing a political agreement by 1 March 2015, with all-important technical details to be sealed by 1 July.
How did the crisis start?
Iran's nuclear programme became public in 2002, when an opposition group revealed secret activity including construction of an uranium enrichment plant and a heavy-water reactor. Enriched uranium can be used to make reactor fuel but also nuclear weapons, while spent fuel from a heavy-water reactor contains plutonium suitable for a bomb.
Iran subsequently agreed to inspections by the global nuclear watchdog, the International Atomic Energy Agency (IAEA). But the IAEA was unable to confirm Iran had not sought to develop nuclear weapons.
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, putting pressure on Iran to negotiate.
What do the world powers want?
The US says any agreement should ensure it would take Iran at least a year to make enough fissile material for a nuclear warhead if it chose to do so. Such a limit on the so-called "break-out time" would require a sharp reduction in Iran's uranium capacity.
The US is believed to want Iran's enrichment capacity to equate to the production capability of less than 4,500 of its inefficient, first-generation centrifuges. Iran currently has about 10,000 operational centrifuges, out of a total of 19,000 installed.
The P5+1 also wants Iran to limit its research and development activities, which could enhance centrifuge efficiency. It believes the enrichment restrictions should remain in place for at least two decades and be backed-up by extensive monitoring.
What does Iran say?
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 its 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, Iran says it would ship almost all its stock of low-enriched uranium to Russia and accept more intrusive inspections by the IAEA.
The P5+1 has noted that Russia, which recently agreed to build two new reactors in Iran, is prepared to supply fuel for Bushehr for its lifetime.
What other obstacles are there to a deal?
Domestic political constraints are being blamed for hindering a compromise on enrichment.
Analysts say Iranian negotiators would struggle to defend an agreement that does not preserve their country's current capacity. Western negotiators would likewise struggle to sell a deal that allowed Iran to rapidly produce weapons-grade uranium.
Israel's Prime Minister Benjamin Netanyahu has said any deal would need to see Iran halt all enrichment activity.
There has also been disagreement over how and when sanctions imposed on Iran would be lifted.
Iran wants UN sanctions to be lifted quickly. However, the P5+1 believes that should happen in the final phase of any accord because they could not be quickly or easily reimposed.
President Barack Obama can suspend sanctions imposed by the US Congress, but he cannot lift them permanently, which might dissuade companies from resuming trade with Iran.
Why is Iran suspected of seeking nuclear weapons?
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 2009 Iran's Supreme Leader, Ayatollah Ali Khamenei, declared: "We fundamentally reject 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, which the IAEA has been unable to visit since 2005.
In September 2014, the IAEA said Iran had failed to give a satisfactory explanation of its research at Parchin into detonators that could be used to trigger a nuclear weapon or explained studies that could help calculate the explosive yield of one.
How does Iran justify its refusal to obey UN resolutions?
Iran has said it is simply doing what it is allowed to do under the NPT, 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 said in April 2014 that Iran had the ability to produce enough weapons-grade uranium for a nuclear bomb within two months.
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 Iran had converted all of its medium-enriched uranium into forms that were less of a proliferation risk.
Any bomb-making process would likely be done in secret, so estimating timelines would be extremely difficult.