A chronology of key events:

1953 US and British intelligence services help Iranian military officers depose Prime Minister Muhammad Mussadeq, a leading exponent of nationalising the oil industry.

Ayatollah Khomeini's legacy still overshadows US-Iranian relations

1979 16 January - US-backed Shah of Iran forced to leave the country after widespread demonstrations and strikes.

1979 1 February - Islamic religious leader Ayatollah Khomeini returns from exile and takes effective power.

1979 4 November - Iranian students seize 63 hostages at US embassy in Tehran, prompting drawn-out crisis leading to severing of diplomatic ties and sweeping US sanctions against Iran. Their initial demand is that the Shah return from the US to Iran to face trial. Later Iran also demands the US undertake not to interfere in its affairs.

1980 25 April - Secret US military mission to rescue hostages ends in disaster in sandstorm in central Iranian desert.

1980 27 July - Exiled Shah dies of cancer in Egypt, but hostage crisis continues.

1980 22 September - Iraq invades, sparking a war with Iran which lasts the rest of the decade. While several Western countries provide support to Iraq during the war, Iran remains diplomatically isolated.

1981 20 January - Last 52 US hostages freed in January after intense diplomatic activity. Their release comes a few hours after US President Jimmy Carter leaves office. They had been held for 444 days.

1985/6 US holds secret talks with Iran and makes weapons shipments, allegedly in exchange for Iranian assistance in releasing US hostages in Lebanon. With revelations that profits were illegally channelled to Nicaraguan rebels, this creates the biggest crisis of Ronald Reagan's US presidency.

1987/8 US forces engage in series of encounters with Iranian forces, including strikes on Gulf oil platforms.

The hostage crisis at the US embassy in Tehran lasted 444 days

1988 3 July - US cruiser Vincennes mistakenly shoots down Iran Air Airbus over the Gulf, killing all 290 people on board.

1989 3 June - Ayatollah Khomeini dies. President Khamenei is appointed supreme leader the following day.

1989 17 August - Hashemi Rafsanjani sworn in as president, with apparent backing of both conservatives and reformers in the leadership.

1990/91 Iran remains neutral in US-led intervention in Kuwait. Rapprochement with West hindered by Ayatollah Khomeini's 1989 religious edict ordering that British author Salman Rushdie be killed for offending Islam in one of his novels.

1992/3 Iran criticises perceived US regional interference in the wake of the Gulf War and the 1993 Israeli-Palestinian peace agreement.

1993 US President Bill Clinton takes office.

1995 President Clinton imposes oil and trade sanctions on Iran for alleged sponsorship of "terrorism", seeking to acquire nuclear arms and hostility to the Middle East process. Iran denies the charges.

1996 Mr Clinton stiffens sanctions with penalties against any firm that invests $40m or more a year in oil and gas projects in Iran and Libya.

The Khatami presidency has not led to closer US-Iranian relations

1997 23 May - Muhammad Khatami elected president of Iran.

1998 President Khatami calls for a "dialogue with the American people" in American TV interview. But in a sermon a few weeks later he is sharply critical of US "oppressive policies".

1999 Twentieth anniversary of US embassy siege. Hardliners celebrate the occasion, as reformists look to the future rather than the past.

2000 18 February - Iranian reformists win landslide victory in general election. Shortly afterwards, President Clinton extends ban on US oil contracts with Iran, accusing it of continuing to support international terrorism.

2000 March - US Secretary of State Madeleine Albright calls for a new start in US-Iranian relations and announces lifting of sanctions on Iranian exports ranging from carpets to food products. Iranian foreign ministry initially welcomes the move, but Ayatollah Khamenei later describes it as deceitful and belated.

2000 September - Mrs Albright meets Iranian Foreign Minister Kamal Kharrazi at UN in New York - the first such talks since diplomatic ties were severed in 1979.

2001 June - The US alleges that elements within the Iranian Government were directly involved in the bombing of an American military base in Saudi Arabia in 1996. Tehran angrily rejects the allegations.

Bush branded Iran as part of an "axis of evil"

2001 September - Report by Central Intelligence Agency accuses Iran of having one of the world's most active programmes to acquire nuclear weapons. The CIA report says Iran is seeking missile-related technology from a number of countries including Russia and China.

2002 29 January - US President George W Bush, in his State of the Union address, describes Iran, Iraq and North Korea as an "axis of evil". He warns that the proliferation of long-range missiles being developed in these countries is as great a danger to the US as terrorism. The speech causes outrage in Iran and is condemned by reformists and conservatives alike.

2002 September - Russian technicians begin construction of Iran's first nuclear reactor at Bushehr despite strong objections from US.

2002 December - The US accuses Iran of seeking to develop a secret nuclear weapons programme and publishes satellite images of two nuclear sites under construction at Natanz and Arak.

2003 February-May - The UN's International Atomic Energy Agency (IAEA) conducts a series of inspections in Iran. The country confirms that there are sites at Natanz and Arak under construction, but insists that these, like Bushehr, are designed solely to provide fuel for future power plants.

2003 June - White House refuses to rule out the "military option" in dealing with Iran after IAEA says Iran "failed to report certain nuclear materials and activities". But IAEA does not declare Iran in breach of Nuclear Non-Proliferation Treaty.

Iran's account of its nuclear programme failed to satisfy the US

2003 September - Washington says Iran is not complying with non-proliferation accords but agrees to support proposal from Britain, France and Germany to give Iran until end of October fully to disclose nuclear activities and allow surprise inspections.

2003 October-November - Tehran agrees to suspend its uranium enrichment programme and allow tougher UN inspections of its nuclear facilities. An IAEA report says Iran has admitted producing plutonium but adds there is no evidence that it was trying to build an atomic bomb. However, US dismisses the report as "impossible to believe". The IAEA votes to censure Iran but stops short of imposing sanctions.

2003 December - US sends humanitarian aid to Iran after earthquake kills up to 50,000 people in city of Bam. US Deputy Secretary of State Richard Armitage and Iran's permanent envoy to UN, Mohammad Javad Zarif, hold telephone talks in a rare direct contact.

2004 January - President Bush denies that US has changed its policy towards Tehran and says moves to help Iran in the wake of earthquake do not indicate a thaw in relations.

2004 March - A UN resolution condemns Iran for keeping some of its nuclear activities secret. Iran reacts by banning inspectors from its sites for several weeks.

2004 September - The IAEA passes a resolution giving a November deadline for Iran to suspend uranium enrichment. Iran rejects the call and begins converting raw uranium into gas.

A US nuclear monitor publishes satellite images of an Iranian weapons facility which it says may be involved in work on nuclear arms.

2004 November - Iran agrees to a European offer to suspend uranium enrichment in exchange for trade concessions. At the last minute, Tehran backs down from its demand to exclude some centrifuges from the freeze. The US says it maintains its right to send Iran unilaterally to the UN Security Council if Tehran fails to fulfil its commitment.

2005 January - Europe and Iran begin trade talks. The European trio, France, Germany and the UK, demand Iran stop its uranium enrichment programme permanently.

2005 February - Iranian President Mohammed Khatami says his country will never give up nuclear technology, but stresses it is for peaceful purposes. Russia backs Tehran, and signs a deal to supply fuel to Iran's Bushehr reactor.

New US Secretary of State Condoleezza Rice says attacking Iran is not on the US agenda "at this point in time".

2005 March - President George W Bush signals a major change in policy towards Iran. He says the US will back the negotiation track led by the European trio - EU3 - and offer economic incentives for the Islamic state to give up its alleged nuclear ambitions.

Mr Bush announces the US will lift a decade-long block on Iran's membership of the World Trade Organization, and objections to Tehran obtaining parts for commercial planes.