Senior diplomats from Iran were due to meet with six leading world powers here on Saturday for talks on the Islamic Republic's nuclear programme.

Representatives of the United States, China, Russia, Britain, France and Germany will be seeking to persuade Iran to scale back its enrichment of uranium, which can be used for peaceful purposes but also for a nuclear bomb.

They also want Iran to grant the UN atomic agency greater access to its nuclear sites and to ship its enriched uranium stockpiles abroad. Iran insists on its right to peaceful nuclear activities and wants sanctions to be eased.

Herewith key developments in the standoff over Iran's nuclear programme:

2005

- Aug 8: Iran resumes uranium conversion activities which had been suspended since November 2004.

2006

- April 11: Iran says it has enriched its first uranium to 3.5 percent purity and later, in May, to 4.8 percent. This is insufficient from the 90-percent level needed for a nuclear bomb but shows Iran is making progress.

- Dec 23: The UN Security Council imposes sanctions on Iran's trade in sensitive nuclear materials and technology. It strengthens the measures in 2007 and 2008.

2007

- April 9: President Mahmoud Ahmadinejad says Iran can produce nuclear fuel on an industrial scale.

2009

- April 9: Iran inaugurates its first nuclear fuel plant, and says it has installed 7,000 uranium enrichment centrifuges at Natanz.

- Sept 25-28: Iran reveals a secret uranium enrichment plant, Fordo, which is being built inside a mountain near the holy city of Qom.

2010

- June-July: World powers enact new military and financial sanctions.

- Aug 21: Iran starts loading fuel into its Russian-built first nuclear plant at Bushehr.

2011

- Jan 22: Failure of new talks between Tehran and six world powers in Istanbul.

- July 19: Iran says it has begun installing new centrifuges with better quality and speed.

- Aug 22: Iran says it has begun transferring centrifuges from Natanz to the Fordo underground site.

- Nov 8: The IAEA publishes a major report citing "overall, credible" evidence from different sources that until the end of 2003, and possibly since, Iran had "activities... relevant to the development of a nuclear explosive device."

- Dec 31: US President Barack Obama signs into law tough new sanctions targeting Iran's central bank and financial sector.

2012

- Jan 1: Iran says its scientists have "tested the first nuclear fuel rod produced from uranium ore deposits inside the country."

- Jan 9: The IAEA confirms that Iran has started enriching uranium at Fordo to purities of 20 percent, a major technological step that takes it significantly closer to being able to enrich to weapons-grade, if it decided to do so.

- Jan 11: An Iranian nuclear scientist is killed in a Tehran car bomb assassination, the fourth scientist killed in two years.

- Jan 23: The EU announces a ban on Iranian oil, along with sanctions against Iran's central bank.

- Feb 15: Ahmadinejad unveils what is said to be Iran's first domestically produced, 20-percent enriched nuclear fuel for Tehran's research reactor and says 3,000 more centrifuges had been added to the uranium enrichment effort.

Iran also activated a new generation of centrifuges that it said increased its enrichment capacity by three times.

- Feb 22: The chief UN nuclear inspector returns from a second visit to Iran, saying his team was not given access to the Parchin military site where Tehran is alleged to have carried out tests aimed at developing nuclear warheads

- March 6: Global powers offer to resume talks with Tehran.

- April 14: World powers and Iran set to meet again in Istanbul to discuss Tehran's promised "new initiatives" on its nuclear activities.