Here is a list of major developments in the diplomatic crisis over Iran's nuclear programme over the past year:
Dec 13 - The United States accuses Iran of launching a secret nuclear weapons programme, publishing satellite images of two sites under construction in the towns of Natanz and Arak. Iran denies any military purpose behind its nuclear activities and agrees to IAEA inspections.
Feb 21 - IAEA chief Mohamed ElBaradei arrives in Iran to carry out inspections at Iran's nuclear sites. From then until May, the UN watchdog carries out a series of inspections in Iran, while Tehran maintains that its nuclear sites are designed purely to provide nuclear fuel for future power plants.
June 16 - EU foreign ministers and ElBaradei call on Iran to allow tougher nuclear inspections, as a report by the IAEA chief says Tehran failed to report nuclear activities, including the purchase of uranium.
July 19 - An IAEA team begins a fresh round of inspections, while the Washington Post newspaper reveals that UN nuclear experts had discovered traces of enriched uranium -- which can be used to make weapons -- at a plant in Natanz during earlier checks.
July 21 - The European Union warns it may review its relations and economic ties with Iran unless Tehran cooperates fully with the IAEA.
Aug 4 - IAEA experts begin fresh talks with Iran on the holding of surprise nuclear inspections
Aug 6 - Iranian President Mohammad Khatami says nuclear weapons have no place in his country's political or military strategy, but the country will not give up nuclear technology.
August 26 - ElBaradei confirms that inspectors at the Natanz facility had found traces of highly enriched uranium -- whose usefulness for non-military purposes is questioned by the agency. Iran agrees to negotiate on a draft protocol allowing surprise inspections but says inspectors will not be given complete freedom of movement.
Sept 9 - Washington says Tehran is in "non-compliance" with international non-proliferation accords, but agrees to support a proposal by Britain, Germany and France to give Tehran until October 31 to fully disclose its nuclear activities and allow surprise inspections of all sites by UN inspectors.
Sept 12 - The IAEA goes ahead and gives Iran until October 31 to prove that it is not developing a nuclear weapon, but Iran refuses the ultimatum.
Oct 2 - IAEA experts begin a new round of talks with Iranian officials ahead of another series of inspections.
Oct 6 - Tehran expresses its good faith, announcing its intention to comply with certain demands of the ultimatum.
Oct 14 - An Iranian opposition group claims in Vienna that Iran has built a uranium-enrichment facility in Isfahan for military purposes.
Oct 18 - IAEA and Iranian experts begin talks leading toward an eventual signature of the protocol.
Oct 21 - The foreign ministers of Britain, France and Germany receive a commitment from Tehran for "total transparency" over its nuclear activities.
Oct 23 - Iran files a report to the IAEA admitting to failures in honoring international nuclear safeguards, but still denying that it seeks to develop nuclear weapons.
Nov 26 - The IAEA, torn between demands for strong action from the United States and a more moderate European attitude, adopts a compromise criticizing Iran but stopping short of calling for a UN Security Council meeting on the nuclear issue.
Dec 17 - The IAEA announces that Iran will sign a protocol allowing UN inspectors to stage surprise inspections of suspect nuclear sites.
Dec 18 - Iran's ambassador to the IAEA Ali Akbar Salehi signs the protocol at the UN watchdog's headquarters in Vienna.