Iran, the United States and five other major powers are labouring to reach an agreement by Tuesday to restrain the Iranian nuclear programme in exchange for relief from economic sanctions.
The deal seeks to allay suspicions that Iran is pursuing a nuclear weapons capability, something Tehran denies, saying its programme is for purely peaceful purposes.
Below is a chronology of Iran's nuclear programme and efforts to constrain it.
1957 – The United States and Iran, then ruled by Shah Mohammad Reza Pahlavi, who was brought to power by a 1953 CIA coup, sign an agreement on civil nuclear cooperation.
1967 – The United States provides Iran with the Tehran Research Reactor (TRR), a 5-megawatt nuclear reactor along with weapon-grade 93 percent enriched uranium fuel.
1968 – Iran signs the Nuclear Non-Proliferation Treaty (NPT) which, when ratified two years later, permits Iran to have a civil nuclear programme in return for a commitment not to acquire nuclear weapons.
1974 – Iran's safeguards agreement with the International Atomic Energy Agency (IAEA) enters into force, giving the U.N. nuclear watchdog the ability to verify its compliance with the NPT.
1974 – Iran hires Germany's Siemens to build a nuclear power plant at Bushehr.
1979 – The shah is toppled during the Islamic Revolution, which brings clerical rulers to power in Tehran. As a result, Iran is no longer able to import 93 percent enriched uranium from the United States or Europe.
1979 – Siemens workers flee revolutionary Iran. Work on the Bushehr atomic power plant stops and Iran's nuclear programme goes dormant.
1980 – Start of the Iran-Iraq war, during which Iraq bombed Bushehr.
1987 – Argentina is hired to convert the TRR to operate on 20 percent enriched uranium fuel, which Iran can more easily procure.
1987 – Iranian officials establish contact with an illicit nuclear procurement network run by Adbul Qadeer Khan, a Pakistani nuclear engineer. The Khan network helps Iran acquire uranium-enrichment technology.
2002 – An Iranian exile group opposed to the government in Tehran reveals that Iran had two previously undisclosed nuclear facilities under construction: a uranium-enrichment plant at Natanz and a heavy water-moderated nuclear reactor at Arak.
Uranium can be enriched to make fuel for power plants or, if purified further, fissile material for bombs. Heavy water reactors yield spent fuel including plutonium, another fissile material.
2003 – In negotiations with Britain, France and Germany, a group known as the EU3, Iran agrees to suspend uranium- enrichment activities and to sign the Additional Protocol, a voluntary pact giving the IAEA greater access to nuclear sites.
2005 – After hardliner Mahmoud Ahmadinejad is elected Iran's president, EU3-Iran negotiations break down and Iran resumes enrichment activities.
2006 – IAEA board of governors refers Iran's case to U.N. Security Council for failure to comply with its Safeguards Agreement.
2006 – The United States ends its opposition to engagement with Iran on the nuclear issue and joins the three EU powers, along with Russia and China, a group known as the "P5+1" and the "E3+3".
2006 – The U.N. Security Council for the first time imposes sanctions on Iran for refusing to suspend enrichment and other sensitive nuclear work.
2008 – U.S. President George W. Bush for the first time sends an official, then-Under Secretary of State Bill Burns, to directly take part in nuclear negotiations with Iran in Geneva.
2009 – U.S. President Barack Obama takes office and, in his first week in the White House, tells Iran's leaders he would extend a hand if they would "unclench their fist" and persuade the West they were not trying to build a nuclear bomb.
2009 – Britain, France and the United States announce that Iran is building a secret uranium-enrichment site at Fordow, near the Shi'ite Muslim holy city of Qom. Iran says it disclosed the site to the U.N. nuclear watchdog earlier in the week.
2009-2012 – Negotiations between major powers and Iran largely stall.
2010 – U.N. Security Council adopts resolution 1929, which creates a basis for crippling U.S. and European Union sanctions in subsequent years.
2012 – U.S. law goes into effect giving Obama the power to sanction foreign banks, including the central banks of U.S. allies, if they fail to significantly reduce their imports of Iranian oil. The result is a drastic reduction in Iranian oil sales and a sharp downturn in the Iranian economy.
2012 – U.S. and Iranian officials begin secret talks, which intensify in 2013, on the nuclear issue.
2013 – Pragmatist Hassan Rouhani is elected Iran's president on platform of improving Iran's relations with the world and the sorry state of its economy, something that can only be achieved by easing sanctions imposed because of Iran's nuclear programme.
2013 – Washington and Tehran intensify their nuclear talks, with U.S. officials using U.S. military aircraft, hotel side entrances and service elevators to keep their role under wraps.
2013 – On Nov. 23, with the groundwork laid by the secret U.S.-Iran talks, Iran and the six major powers reach an interim pact called the Joint Plan of Action (JPOA) under which Iran agreed to curb its nuclear work in exchange for limited sanctions relief.
The aim was to reach and implement a comprehensive agreement within a year, but the deal and deadline have been extended three times.
2015 – On April 2, Iran and the major powers, Britain, China, France, Germany Russia and the United States reach the framework for a Comprehensive Joint Plan of Action (CJPOA) or final agreement.
Under the framework, as described by the United States, Iran committed to reduce the number of centrifuges it operates by about two-thirds and to other long-term limitations on its nuclear activities.
On June 30, both sides agree to give themselves until July 7 to reach a final deal, though a senior U.S. official says talks could run beyond that if they are on the verge of an accord.
