Iran says it has made significant advances in its nuclear programme, building new uranium enrichment centrifuges and producing its own nuclear reactor fuel plates.
State media said on Wednesday that Iranian President Mahmoud Ahmadinejad has inserted the first Iranian-manufacured fuel rod into a Tehran test reaction. Footage showed Ahmadinejad, wearing a white lab coat, being briefed by scientists.
Iran had added 3,000 more centrifuges to its uranium enrichment effort, Ahmadinejad said in a speech broadcast on state television.
"Approximately 6,000 centrifuges were working, 3,000 have been added to that amount. [Now] there are 9,000," he said.
The claimed breakthrough is likely to further unsettle the US and allies who believe Iran is attempting to acquire nuclear weapons. Iran says its nuclear programme is for peaceful purposes.
Iran has developed "fourth generation centrifuges" made of carbon fibre that are "speedier, produce less waste and occupy less space" as they spin at supersonic speeds to purify uranium, according to Wednesday's announcement.
Iran also created its own 20 per cent fuel plates for the research reactor, whose stock of fuel, sourced from Argentina in the 1990s, is running low, the report said.
State television also said that Iran had made progress in 20 per cent uranium enrichment at its Natanz facility, beyond enrichment activities already under way there.
Al Jazeera's Dorsa Jabbari, reporting from Tehran, said state television showed Ahmadinejad; Fereydoon Abassi Davani, head of Iran's Atomic Energy Organisation; and other officials watching a metal-encased rod being inserted into the facility's pool.
"This is a huge achievement for the Iranian people because these fuel rods are actually produced domestically. It is the first batch that have been transferred to any of the facilities to start work on medical isotopes," she said.
US officials played down Iran's claims, with Jay Carney, the White House press spokesman, calling them "provocative" and "defiant".
He said the announcement was intended to distract attention from the impact on Iran of US-led sanctions.
"It is not unusual for Iran to try to distract attention from those uncomfortable facts and from its overall isolation by some burst of rhetoric or making some announcement," Carney said.
State Department spokeswoman Victoria Nuland said the Iranian uranium enrichment advance was "not terribly new, and not terribly impressive."
Iran's letter to EU
Iran earlier replied to a letter sent nearly four months ago by Catherine Ashton, the EU foreign policy chief, proposing a resumption of stalled talks with world powers on its nuclear programme, the official IRNA news agency reported.
The letter, written by Saeed Jalili, Iran's chief nuclear negotiator, and handed to Ashton's office, reads: "Iran welcomes the readiness of the P5+1 group to return to negotiations in order to take fundamental steps toward further co-operation."
The P5+1 consists of the five permanent UN Security Council members - Britain, China, France, Russia and the US - plus non-permanent member Germany.
Unprecedentedly tough US and EU economic sanctions have been imposed on Iran in recent weeks and months, in an attempt to push it to halt its nuclear programme.
In response, Iran has threatened to retaliate by closing the Strait of Hormuz, the main Gulf oil shipping lane.
The UN nuclear watchdog expressed strong suspicions in November that Iran's programme had a military component.
It is to send a high-level delegation back to Iran next week to discuss concerns, after inconclusive talks in Tehran late last month.
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