TEHRAN - Iran said on Wednesday it viewed pending nuclear talks with six world powers as an opportunity and a test, and the United States said an extraordinarily difficult process was starting with more meetings likely.

Washington has said big powers will pursue harsher sanctions on Iran over its nuclear program if the new dialogue fails.

This, from the point of view of the United States, cannot be an open-ended process, or talks just for the sake of talks, especially in light of the revelations about Qom, said a senior U.S. official, who declined to be identified.

He was referring to a uranium enrichment site near Qom in Iran, which Tehran revealed to the U.N. nuclear watchdog last week after, according to Western diplomats, learning that the site had been detected by Western intelligence services.

As Iran's chief nuclear negotiator left for Switzerland expressing goodwill, the head of the U.N. nuclear watchdog said Tehran had broken a transparency law by failing to disclose much earlier its second enrichment site.

Iran reported the site to Mohamed ElBaradei's International Atomic Energy Agency (IAEA) on September 21, 3-1/2 years after Western powers said construction of the plant began.

Iran was supposed to inform us on the day it was decided to construct the facility. They have not done that, ElBaradei said in an interview with CNN-India during a visit to New Delhi, in remarks relayed by the IAEA's Vienna headquarters.

With Iran ruling out any discussion in Geneva about its own atomic program, which the West suspects is aimed at making bombs, there was little sign that Thursday's session would lead to any breakthrough in the long-running dispute.

Iran has offered wide-ranging security talks while making clear its nuclear rights will be off-limits. It says its nuclear technology is to generate electricity, not make bombs.

The United States and its Western allies have made clear they will focus on Iran's nuclear activities at the first such meeting since U.S. President Barack Obama took office.

While Iran and the six powers -- the United States, China, Russia, Britain, France and Germany -- prepared for talks, U.S. and British officials seemed to differ over Iran's capabilities.

A British security source said London suspected Iran had been seeking nuclear weapons for the past few years, in contrast to a U.S. view that Tehran halted work on design and weaponization in 2003. We didn't share the U.S. assessment and still do not, the source said.

ElBaradei said he had no evidence to back up the British assessment.

U.S. officials are focusing for now on diplomatic efforts, but the White House is considering sanctions targeting Iran's dependence on gasoline imports and insurance firms that underwrite the trade.

Obama warned Iran last week to come clean about its nuclear work or face sanctions that bite.

He said the Geneva talks represented an exceptional opportunity for (Western countries) to change their situation in the world and correct their way of dealing with nations.

These talks could be a test to verify whether some governments are determined to follow up the slogan of change, Ahmadinejad said according to IRNA news agency, referring to Obama.

Ahmadinejad proposed an organized structure for the discussions, with three committees dealing with different issues, and an assembly of heads of states of the countries involved as the top decision-making body, Fars News Agency said.

We are entering the talks with goodwill, chief nuclear negotiator Saeed Jalili said at Tehran's international Imam Khomeini airport.

Washington has suggested possible new sanctions on banking and the oil and gas industry if Iran, the world's fifth-largest crude exporter, fails to assuage Western fears it seeks nuclear weapons.

Ahmadinejad said Iran had prepared itself for all possibilities: The Iranian nation has learnt to stand on its own feet during the past 30 years.

On Tuesday, an Iranian MP, Mohammad Karamirad, was quoted as saying parliament may advocate Iran's withdrawal from the nuclear Non-Proliferation Treaty (NPT) if talks with big powers fail and the United States keeps putting pressure on Tehran.

Another MP, Hassan Ghafourifard, made similar remarks in an interview carried by state Press TV on Wednesday.

Parliament can formally oblige the government to take such a step, as happened when Iran stopped permitting wide-ranging snap U.N. nuclear inspections in 2006, but Supreme Leader Ayatollah Ali Khamenei has the final say on matters of state.

Top Iranian officials have repeatedly said Tehran has no intention to leave the NPT, under which its nuclear facilities are subject to regular U.N. nuclear watchdog inspections, or seek nuclear weapons it says violate the tenets of Islam.

Analysts believe Iran would think twice before quitting the NPT since such a move would betray nuclear weapons ambitions and could provoke pre-emptive attack by Israel and possibly the United States.