Here is a summary of the main developments in the Iran nuclear crisis since 2002:
2002-2004: Undeclared nuclear sites
After nuclear sites are uncovered in Natanz and Arak in central Iran in August 2002, Tehran agrees to undergo an inspection by the International Atomic Energy Agency (IAEA). The UN watchdog reveals that traces of enriched uranium have been identified.
On October 21st 2003, following an unprecedented visit by foreign ministers from Britain, France and Germany, Iran suspends uranium enrichment activities but later vows it will never renounce its nuclear programme.
2005-2008: Enrichment to 3.5 percent
On August 8, 2005, after the election of hardline president Mahmoud Ahmadinejad, Tehran resumes uranium enrichment. European nations break off negotiations.
In January 2006, the UN Security Council's five permanent members — Britain, China, France, Russia and the United States — agree to have the IAEA present the issue to the full council.
They are defied by Iran, which on April 11th says that it has enriched uranium to 3.5 percent purity. It then rejects an offer from the permanent UN members plus Germany, dubbed P5+1, on a framework for talks.
On December 23rd 2006 the UN imposes the first of several rounds of sanctions on Iran's trade in sensitive nuclear materials and technology. It is followed by the United States and European Union.
Negotiations come to a standstill. On November 7th 2007 Iran says it has at least 3,000 centrifuges for enrichment, which in theory would allow it to produce enough enriched uranium for a nuclear bomb in less than a year. Today it has nearly 20,000, of which half are active.
2009-2012: Enrichment to 20 percent
In 2009, following the November election of US President Barack Obama, world powers offer to resume negotiations with Tehran, which later declares major advances in its nuclear drive as Ahmadinejad opens a uranium conversion facility on April 9th in Isfahan.
Then, on September 25th, Western nations reveal the existence of a previously undeclared enrichment site inside a mountain in Fordo.
On February 9th 2010, after the breakdown of talks on enriching uranium in a third country, Iran says it has begun to enrich uranium to 20 percent at Natanz, close to the level required for a nuclear weapon.
With Israel threatening a preventive strike, the IAEA on November 8th 2011 points to a possible military dimension to Iran's nuclear activities.
On January 9th 2012, the IAEA says that Iran has started to enrich to 20 percent at Fordo.
On January 23rd 2012, the EU agrees to slap an embargo on Iranian oil exports and freezes the assets of Iran's central bank.
Negotiations between Iran and the P5+1 resume in April after being stopped for 15 months.
2013: Preliminary accord
On August 6th newly-elected Iranian President Hassan Rouhani says Tehran is ready for "serious" negotiations.
On September 27th Rouhani reveals he and Obama have spoken by telephone in the highest-level contact between the two countries since 1979.
On November 24th after intense negotiations Iran agrees to curb its nuclear programme in exchange for sanctions relief in an interim deal which comes into force on January 20th.
2014: Extended negotiations
On February 18th, negotiations start on a permanent accord, but despite intense diplomatic efforts fail and deadlines are extended twice.
The interim accord is extended. Iran says on August 27th it has started to modify the design its Arak heavy water reactor and limit plutonium output.
2015: Towards a permanent accord?
Negotiations restart in January with a deadline of a political accord by March 31st.
The framework deal, if it can be hammered out, is meant to be finalized with comprehensive technical annexes by June 30th.
In the United States, Republicans take control of the Congress and threaten Iran with preventative sanctions, which Obama vows to veto.
On March 3rd, Israel steps up its campaign against a "dangerous" deal with Israeli Prime Minister Benjamin Netanyahu blasting the accord in a controversial speech to Congress.
On March 26th, top US diplomat John Kerry and Iranian Foreign Minister Mohammad Javad Zarif meet in Lausanne at the start of a flurry of talks for the outlines of a political accord.