Herewith are key dates in the crisis:

2002

-- January: US President George W. Bush accuses Iran, along with Iraq and North Korea, of forming an "axis of evil."

-- August: Iranian exiles say Tehran has built nuclear plants without informing the United Nations.

-- December: Satellite photos reveal the existence of sites; Iran agrees to inspections by the UN's International Atomic Energy Agency, insisting its nuclear work is only for legitimate energy needs.

2003

-- February: IAEA chief Mohamed ElBaradei travels to Iran with a team of experts.

-- August: A UN report says traces of enriched uranium have been found at an Iranian site.

-- October: After talks with Britain, France and Germany, Iran agrees to suspend uranium enrichment and to sign an additional protocol to the Nuclear Non-Proliferation Treaty, allowing tougher inspections.

2004

-- June: The IAEA complains Iran is not providing "full, timely and pro-active" cooperation with inspectors.

-- November: Iran agrees to suspend uranium enrichment and reprocessing activities while new talks are under way with Britain, France and Germany.

2005

-- August: Mahmoud Ahmadinejad is installed as Iranian president. Iran resumes uranium conversion activities and the EU breaks off talks.

-- September: The IAEA finds Iran in violation of the NPT and says it reserves the right to send the case to the Security Council.

2006

-- January: Iran breaks IAEA seals at its Natanz nuclear research facility; the following month it resumes uranium enrichment there.

-- July: The UN Security Council demands Iran halt work on nuclear fuel, threatening sanctions for the first time.

-- December: The UN Security Council votes to impose sanctions on Iran's trade in sensitive nuclear materials and technology. Iran vows to press ahead with enrichment.

2007

-- January: Iran says it has blocked entry to 38 IAEA inspectors in retaliation for sanctions.

-- February: The IAEA says Iran has expanded its work.

-- March: The UN Security Council adopts further sanctions, while offering more incentives to end the nuclear stand-off.

-- April: Ahmadinejad says Iran can produce nuclear fuel on an industrial scale.

-- October: Ali Larijani resigns as top Iranian negotiator and is replaced by Deputy Foreign Minister Said Jalili.

-- November: the IAEA finds that while Iran has been telling the truth about key aspects of its past nuclear activities, its knowledge of Tehran's current nuclear activities is diminishing.

-- December: The US National Intelligence Estimate on Iran says Tehran abandoned a project to develop nuclear weapons in 2003 and has not resumed it.

2008

-- January: World powers reach agreement on a new UN resolution against Iran. The IAEA and Iranian leaders agree to a four week deadline for Iran to clear up remaining questions on its nuclear program.

-- February: The UN atomic watchdog says it has made "quite good progress" in its probe, but is still not in a position to offer a verdict on Tehran's nuclear ambitions.