President Ahmadinejad: "We have no secrecy, we work within the framework''
Iran's newly-revealed nuclear facility is open for inspection by UN experts, the country's president has said.
Mahmoud Ahmadinejad insisted that the plant, thought to be under construction near the holy city of Qom, was being built in line with UN regulations.
Leaders of the US, UK and France have accused Iran of building a new plant to enrich uranium, in breach of UN rules.
They raised the prospect of new, tough sanctions against Iran if it does not fully co-operate with global powers.
However, at a news conference in New York, where he has been attending the United Nations General Assembly, Mr Ahmadinejad firmly rebutted the Western criticism.
Those leaders who had complained about the plant had made a big mistake and would regret the move, he said.
"We don't have any problems with inspections of the facility. We have no fears," he said, referring to calls for immediate access to the site by inspectors from the International Atomic Energy Agency (IAEA), the UN watchdog.
Mr Obama is not a nuclear expert. We have to leave it to the IAEA to carry out its duties
He justified Iran's apparent concealment of the plant by saying there were no international requirements to declare any nuclear facility until 180 days before fissile material was introduced into it.
But the BBC's Paul Reynolds says there is a dispute about the amount of notice that Iran is required to give the IAEA before a new nuclear facility becomes operational.
In 2003, Iran agreed on what is called a Subsidiary Arrangement, under which it is required to tell the IAEA at the preliminary design stage. Iran later announced that it had repudiated this agreement, but the IAEA says that no such unilateral repudiation is allowed.
Mr Ahmadinejad also flatly denied claims - by US President Barack Obama, French President Nicolas Sarkozy and UK Prime Minister Gordon Brown - that the plant was a secret facility.
"If it was, why would we have informed the IAEA about it a year ahead of time?" Mr Ahmadinejad was reported as saying.
He reiterated Tehran's position that Iran was not interested in developing nuclear weapons, describing them as "against humanity".
Mr Ahmadinejad spoke out hours after Mr Obama, Mr Sarkozy and Mr Brown took to the stage in Pittsburgh - site of a G20 summit - to condemn the building of the new plant.
Iran's decision to build a secret facility represented a "direct challenge to the basic compact" of the global non-proliferation regime, Mr Obama said.
BBC World Affairs correspondent Paul Reynolds
Iranian ambitions for this site are not known. It could be that they wanted a back-up in case their main plant at Natanz was attacked. But another fear is that they intended to enrich uranium more highly at the secret plant, to a level suitable for a nuclear explosion.
The discovery will strengthen the demands by the US and its allies for further sanctions to be imposed on Iran unless it suspends all enrichment, as required by the Security Council.
The new facility is said to be underground at a mountain on the site of a former missile site belonging to the Iranian Revolutionary Guards, north-east of the holy city of Qom.
Construction on the facility - believed by the US to be large enough to contain 3,000 centrifuges, not large enough for commercial work - started in earnest in mid-2006, diplomatic sources said.
Iran's letter to the UN watchdog, the IAEA, on Monday informed it that "a new pilot fuel enrichment plant is under construction".
Iran told the agency that no nuclear material had been introduced into the plant, and enrichment levels would only be high enough to make nuclear fuel, not a bomb.
In response, the IAEA requested Iran to "provide specific information and access to the facility as soon as possible".
The disclosure of the new plant comes one day after world leaders stressed the need for greater co-operation against nuclear proliferation and shortly before Iran is due to resume talks with international powers on the issue.
Earlier this month, Tehran agreed to "comprehensive" discussions on a range of security issues - but made no mention of its own nuclear programme.
The talks are due to be held in Geneva on 1 October with Tehran and the five permanent UN Security Council members - US, UK, Russia, China and France - plus Germany.
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