|Iran has denied US accusations that is using the nuclear power plant in Bushehr, built in cooperation with Russia, to weaponise its programme [EPA]
Iran first began experimenting with nuclear energy in the 1950s when it received US assistance to build the country's first nuclear reactor.
At the time, the Eisenhower administration in Washington saw Tehran as a vital ally and pursued a co-operative programme to push US scientific and commercial involvement in Iranian civilian nuclear industries.
In the 1970s, the Iranian government sought - and received - assistance from other western nations such as Germany and France.
Despite US intelligence warnings that Iran could have nuclear weaponisation ambitions, Western assistance continued.
However, the overthrow of the Shah in 1979 and the rise to power of Islamist leaders like Ayatollah Khomeini brought the co-operation to an end.
As the war with Iraq raged until 1988, Iran resumed its interest in a national nuclear programme.
Following the Gulf War in 1991 and the UN sanctions slapped on Iraq for its alleged weaponisation programme, Iran began to dedicate funds to speed up its research and development of nuclear power.
In 1995, Russia signed several nuclear deals with Iran; development of the nuclear programme continued relatively unhindered until an opposition group revealed that Tehran had overcome several technological obstacles in securing nuclear power.
Below are key dates since 2002 when the West and Iran clashed over its disputed nuclear programme.
August: The exiled opposition National Council of Resistance of Iran reports the existence of an uranium enrichment facility at Natanz and a heavy water plant at Arak.
December: The United States accuses Iran of an "across-the-board pursuit of weapons of mass destruction".
February: International Atomic Energy Agency (IAEA) inspects the Natanz and Arak facilities.
June: An IAEA report on the inspections says that Iran has failed to comply with the nuclear Non-Proliferation Treaty.
|Iran insists that its nuclear programme
is for civilian purposes [EPA]
November: Iran promises EU negotiators it will suspend all nuclear fuel processing and reprocessing work.
September 2: IAEA report says that Iran has resumed uranium conversion at the Isfahan nuclear research facility.
January 10: Iran resumes nuclear fuel research at the Natanz enrichment plant after breaking the UN seals on the facility.
February 4: IAEA votes to report Iran to the UN Security Council.
February 5: Iran ends snap UN nuclear inspections.
April 11: Iran announces it has produced low-grade enriched uranium suitable for use in power stations, a move confirmed by the IAEA.
February 14: Iran restarts small-scale feeding of uranium gas into centrifuges at Natanz after a two-and-a-half year suspension.
June 5: Javier Solana, European Union foreign policy chief, delivers a package of incentives from world powers if Iran agrees to halt uranium enrichment.
August 31: The IAEA announces Iran has not met a deadline to suspend its atomic fuel programme.
December 23: Security Council votes for sanctions and gives Iran a 60-day deadline to suspend enrichment. Iran calls the resolution illegal.
March 24: The Security Council unanimously approves further financial and weapons sanctions against Iran.
April 18: The IAEA says Iran has begun making nuclear fuel in an underground uranium enrichment plant.
May 23: A confidential IAEA report says Iran has not suspended enrichment-related work.
August 21: Iran and the IAEA say they have agreed a timeline for answering the watchdog's outstanding questions about Iran's nuclear programme.
|Saeed Jalili replaced Ali Larijani as chief
nuclear negotiator in October [AFP]
October 20: Saeed Jalili is named as Iran's new chief nuclear negotiator. He replaces Ali Larijani, who resigned.
October 24: The US imposes new sanctions on Iran and accuses the elite Revolutionary Guard of spreading weapons of mass destruction.
November 2: Britain, France, Germany, the US, Russia and China (the group of six) agree to push ahead with a third round of tougher sanctions.
November 15: The IAEA says Iran has made important strides towards transparency but it remains unable to ascertain whether Iran has a secret, parallel military enrichment programme.
November 30: The EU expresses disappointment with Iran after more talks with its main nuclear negotiator in London.
December 1: A meeting of world powers in Paris fails to reach an agreement on further sanctions.
December 3: A US National Intelligence Estimate says Iran halted its attempts to build a nuclear bomb in 2003.
December 5: Victory over the US is declared by Mahmoud Ahmadinejad, the Iranian president.
December 11: Opposition coalition NCRI says that Iran's programme was shut down in 2003 and re-started a year later, adding that the recent US analysis was misleading.
March 3: UN Security Council adopts third sanctions resolution targeted at Iran's nuclear programme.
May 14: Russia's foreign minister says an offer to negotiate and security guarantees for Iran could be given by the group of six. The US administration denies that security guarantees were being considered.
May 26: The IAEA says Iran's alleged research into nuclear warheads is a matter of serious concern and asks for more information on Tehran's missile-related activities.
June 1: In reaction to the IAEA's report, Tehran says it might have to limit cooperation with them.
June 14: Solana, in Tehran, presents Iran with an offer from world powers with economic and other benefits. However, Iran rejects any suspension of activities.
July 10: Iran tests nine missiles in the Gulf.
July 19: Iranian officials rule out freeze in uranium enrichment during talks in Geneva, attended for the first time by a senior US diplomat.
August 2: An informal deadline lapses for Iran to respond to an offer from the United States, Britain, France, Germany, China and Russia for talks.
September 15: The IAEA says Iran blocks a UN inquiry into whether it researched ways to develop a nuclear bomb.
February 5: Russia says it plans to start up a nuclear reactor at the Bushehr plant by the end of 2009.
February 19: An IAEA report shows a significant increase in Iran's stockpile of low-enriched uranium; enough, physicists say, for conversion into highly enriched uranium for one bomb.
|Obama makes a statement on Iran with Sarkozy, left, and Brown at the G20 summit in Pittsburgh [AFP]
March 20: Barack Obama, as the new US president, calls for "engagement that is honest and grounded in mutual respect" with Iran.
Iran cautiously welcomes the overture, but said it wanted to see "practical steps".
April 9: Ahmadinejad says Iran has mastered the nuclear fuel cycle and also tested further advanced machines for enriching uranium. President also inaugurates the nation's first atomic fuel fabrication plant near Isfahan.
June 5: A quarterly IAEA report says Iran now has 7,231 centrifuge enrichment machines installed, a 25 per cent increase in potential capacity since March.
June 12: Ahmadinejad re-elected president. Protests break out by moderates who say the result was fixed.
August 21: Iran allows IAEA officials to inspect the Arak heavy-water site as well as an upgrade to IAEA monitoring at the Natanz uranium enrichment plant. Diplomats say there has been no increase in the number of centrifuges enriching uranium since the end of May.
August 28: The IAEA reports Iran has slightly reduced the scale of its uranium enrichment. But it has also raised the number of installed centrifuge machines by some 1,000 to 8,308.
September 7: Ahmadinejad says Iran will continue its disputed nuclear work and will never negotiate on its "obvious" rights.
September 9: Iran hands over a package of proposals to six world powers - the US, Russia, China, France, Britain and Germany - which it says addresses "various global issues" and represents a "new opportunity for talks and cooperation".
September 17: Ahmadinejad says Iran would "never" abandon its nuclear programme to appease Western critics.
September 25: The IAEA says Iran has told it about a second uranium enrichment plant under construction. Obama announces that the US, UK and France provided evidence to the IAEA showing Iran has been developing a covert uranium enrichment facility near Qom for several years.
September 26: Ahmadinejad reacts strongly to international condemnation of the second uranium enrichment facility, saying it was within the "parameters of the UN nuclear watchdog's rules".
September 27: Hillary Clinton, the US secretary of state, welcomes Iran's decision to allow IAEA inspectors into a newly disclosed uranium enrichment plant.
September 29: Iran test-fires several ballistic missiles.
September 30: Iran says it will not negotiate over its right to develop a nuclear programme when it meets officials from the five permanent members of the UN Security Council and Germany in Geneva on October 1.
October 1: Iran meets six world powers in Geneva. Saeed Jalili, Iran's chief nuclear negotiator, holds talks with William Burns, a senior US official in Geneva, in the most high-level US-Iranian contact in three decades.
October 3: The New York Times reports that staff at the International Atomic Energy Agency have written a confidential analysis conveying that Iran has "sufficient information to be able to design and produce a workable" atom bomb.
October 4: The IAEA announces that its inspectors will examine the Qom plant plant on October 25.
October 21: The IAEA says Iran has agreed to consider a deal which could see it ship out most of its enriched uranium to Russia.
October 23: Iran fails to meet the deadline to accept the IAEA deal.
November 17: Iran rules out sending its enriched uranium abroad for further processing, saying it would prefer to purchase the required fuel from other countries and keep its low-enriched uranium.
November 29: Iran responds to an IAEA resolution demanding that it halt the construction of a newly revealed enrichment facility near the city of Qom by announcing that it will build 10 new nuclear plants.
December 2: The IAEA passes a resolution criticising Tehran for defying a UN Security Council ban on uranium enrichment and rebukes it for secretly building a uranium enrichment facility near the city of Qom. Ahmadinejad rejects the resolution as "illegal".
December 11: The US, Britain and France warn Iran that it risks more sanctions unless it immediately complies with a series of UN Security Council resolutions regarding its nuclear programme.
December 12: The US dismiss an Iranian offer to exchange nuclear fuel, saying it was inconsistent with a deal that would allow Iran to avoid further sanctions.
January 3: The United States calls for fresh sanctions against Iran at the UN Security Council following expiry of year-end deadline to respond to Washington's offer of economic and diplomatic concessions.
January 21: Iranian and Russian officials announce that Iran's first nuclear power plant built with Russian help will be operational by the middle of this year.
January 27: UK-based International Institute for Strategic Studies predicts Iran will have enough low-enriched uranium for a nuclear bomb by end of the year.
February 2: Ahmadinejad gives a speech on state television, saying that Iran would have "no problem" with sending out its stocks of low-enriched uranium under a deal with Western powers.
February 7: In a speech at an exhibition on laser technology, Ahmadinejad tells Ali Akbar Salehi, the head of Iran's nuclear programme, to begin enriching uranium.
March 30: Foreign ministers from the world's eight leading industrial nations call for "appropriate and strong steps" against Iran over its nuclear activities.
April 8: UN ambassadors from the United States, China, Britain, France, Germany and Russia meet in London to discuss imposing new sanctions on Iran for its failure to suspend work on its nuclear programme.
April 9: In a ceremony marking Iran's National Day of Nuclear Technology, Ahmadinejad announces that the country has developed faster centrifuges for uranium enrichment.
Source: Al Jazeera and agencies